Detail:
- Choose the solution that was already in place (before last fix)
  for 6.0
- minor cleanup in comments

- disable the test suffering from Bug#41098 Query Cache returns wrong result with
                                            concurrent insert
- additional improvements

Details:
- Move the test for the historic bug
  Bug 28249 Query Cache returns wrong result with concurrent insert / certain lock
  into its own testscript query_cache_28249.test.
- query_cache.test:
  - replace error numbers with error names
  - remove trailing spaces, replace tabs with spaces
  - reset of @@global.log_bin_trust_function_creators to its original value
    at the end of the test
- query_cache_28249.test:
  - replace sleep 5 with poll routine
  - avoid random differences caused by concurrent_inserts effects
  - improved comments and formatting

(no conflicts)

symlink.test failed when run in an environment that has mysql-test/var
symlinked to elsewhere, e.g. a memory file system. This is the case
when running mysql-test-run --mem.

In this case the server does not detect that the directory specified
with a DATA/INDEX DIRECTORY clause is within its data home directory.
This problem was reported as Bug#39277 (Creation of table with data
and/or index files in data home directory succeeds). It was decided
that it will not be fixed in 5.1. Hence, the current behavior is
accepted for 5.1. It will be fixed in 6.0 though.

Fixed the test case so that it works in both environments. 1. When no
symbolic link is involved, the server notices that the data/index
directory is in its data hone directory and rejects the CREATE/ALTER
TABLE statement. 2. When the data home directory is symlinked, it
does not notice the problem and executes the statement sucessfully.


mysql-test/r/symlink.result:
  Bug#41002 - symlink.test fails on symlinked datadir
  Updated test result.
mysql-test/t/symlink.test:
  Bug#41002 - symlink.test fails on symlinked datadir
  Adjusted the test case to the accepted behavior.
  It needs to accept success and failure of some statements.

Problem: in 5.0 'check table for upgrade' doesn't detect
incompatible collation changes made in 5.0.48.

Fix: backport #39585 fix to 5.0


sql/handler.cc:
  Fix for bug#40984: backport fix from 39585 into 5.0
    - backport of #39585 fix
sql/handler.h:
  Fix for bug#40984: backport fix from 39585 into 5.0
    - backport of #39585 fix

Altering how MTR checks global variable status to exclude timestamp
Changed SQL statements to update style.

Problem was usage of read_range_first with an empty key.

Solution was to not to give a key if it was empty.

mysql-test/r/partition.result:
  Bug#40954: Crash in MyISAM index code with concurrency test using partitioned tables
  
  Updated test result.
mysql-test/t/partition.test:
  Bug#40954: Crash in MyISAM index code with concurrency test using partitioned tables
  
  Added test case

ONLY_FULL_GROUP_BY

The check for non-aggregated columns in queries with aggregate function, but without
GROUP BY was treating all the parts of the query as if they are in the SELECT list.
Fixed by ignoring the non-aggregated fields in the WHERE clause.

mysql-test/r/func_group.result:
  Bug #39656: test case
mysql-test/t/func_group.test:
  Bug #39656: test case
sql/sql_select.cc:
  Bug #39656: ignore the new non-aggregated column refs in a WHERE
  by saving the state so far and then adding only the new values of the other
  parts of the bitmask.

Several system variables did not behave like system variables should do.
When trying to SET them or use them in SELECT, they were reported as
"unknown system variable". But they appeared in SHOW VARIABLES.

This has been fixed by removing the "fixed_vars" array of variables
and integrating the variables into the normal system variables chain.
All of these variables do now behave as read-only global-only
variables. Trying to SET them tells they are read-only, trying to
SELECT the session value tells they are global only. Selecting the
global value works. It delivers the same value as SHOW VARIABLES.


mysql-test/r/variables-notembedded.result:
  Bug#28234 - global/session scope - documentation vs implementation
  New test result.
mysql-test/r/variables.result:
  Bug#28234 - global/session scope - documentation vs implementation
  New test result.
mysql-test/t/variables-notembedded.test:
  Bug#28234 - global/session scope - documentation vs implementation
  Added a test for each moved variable that is not present in an
  embedded server.
mysql-test/t/variables.test:
  Bug#28234 - global/session scope - documentation vs implementation
  Added a test for each moved variable that is also present in an
  embedded server.
sql/item_func.cc:
  Bug#28234 - global/session scope - documentation vs implementation
  Added SHOW_BOOL to some Item_func_get_system_var methods.
sql/set_var.cc:
  Bug#28234 - global/session scope - documentation vs implementation
  Moved all variables from the "fixed_vars" array into the normal
  system variables chain by using the new variable class sys_var_const.
  Removed the fixed_show_vars array and its initialization in
  enumerate_sys_vars().
  Removed mysql_append_static_vars(), which added fixed_vars arrays
  to the fixed_show_vars array.
sql/set_var.h:
  Bug#28234 - global/session scope - documentation vs implementation
  Added the new system variable class sys_var_const.
  Removed declaration of mysql_append_static_vars().
sql/slave.cc:
  Bug#28234 - global/session scope - documentation vs implementation
  Moved the definition of show_slave_skip_errors() from sql_repl.cc
  to here and renamed it to print_slave_skip_errors().
  Changed print_slave_skip_errors() to create a static buffer with
  a printable version of the error numbers set.
  Added a call of print_slave_skip_errors() to init_slave_skip_errors().
sql/slave.h:
  Bug#28234 - global/session scope - documentation vs implementation
  Added declaration of slave_skip_error_names.
sql/sql_repl.cc:
  Bug#28234 - global/session scope - documentation vs implementation
  Moved all variables from the "fixed_vars" array into the normal
  system variables chain by using the new variable class sys_var_const.
  Moved the definition of show_slave_skip_errors() to slave.cc and
  modified it to compute the string once at server initialization only.
  Removed the call to mysql_append_static_vars().

- restore original state of event_scheduler at the end of the test
- minor fixes around comments, formatting

the problem is the same as reported in bug#20835,
so the fix is backport of bug#20835 patch.


mysql-test/r/subselect.result:
  test result
mysql-test/t/subselect.test:
  test case