MDEV-5069: Server crashes in SEL_ARG::increment_use_count with index_merge+index_merge_sort_union, FORCE INDEX
- Don't call incr_refs() is the merged SEL_ARG* is NULL.

Added test case for new system variable innodb_use_stacktrace and made sure that it can be used only on startup. Fixed compiler problems on solaris and other platforms that do not contain necessary headers and functions.

MDEV-5247: DB locked up at btr0cur.c line 568. Additional fixes to inconsistent usage of have_LRU_mutex and added additional debug assertions to guard incorrect usage of this mutex. Fixes issues found on additional testing and mysql test suite.

This bug was a consequence of the incorrect fix for bug mdev-5091.

Fix compiler error introduced on revision 3937, make sure that stackdump is compiled only on __linux__.

mysql_tzinfo_to_sql is not compiled under Windows in the pre-5.5 versions.
Do not set the MYSQL_TZINFO_TO_SQL variable in the version prior to 5.5
under Windows.

MDEV-5247: DB locked up at btr0cur.c line 568. There is inconsistent and non logical usage of have_LRU_mutex and incorrect value on ha_innodb.cc when buf_LRU_free_block is called. Additionally, for future long semaphore wait cases added a new configuration variable innodb_use_stacktrace. If this variable is true a signal handler for SIGUSR2 is installed when InnoDB server starts and when a long semaphore wait is detected at sync/sync0array.c we send SIGUSR2 signal to waiting thread and thread that has acuired RW-latch. For both threads a full stacktrace is produced as well as its is possible.

The function SELECT_LEX::update_used_tables should process all
ORDER BY lists in any subselect of a union.

Allow only one level of symlink recursion in mysql_tzdata_to_sql,
to avoid infinite loops.

Apply fix suggested by Igor:
- When eliminate_item_equal() generates pair-wise equalities from a 
  multi-equality,  do generate a "bridge" equality between the first 
  field inside SJM nest and the field that's first in the overall multi-equality.

MDEV-5113 Wrong result (extra row) and valgrind warnings in Item_maxmin_subselect::any_value on 2nd execution of PS with SELECT subquery

When setting Item_func_not_all::test_sum_item or Item_func_not_all::test_sub_item,
reset the other one to NULL - they can never be set both. When a PS is reexecuted,
different executions might be optimized differently and a wrong test_su*_item
might stay set from the previous execution.

- MIN/MAX optimizer does a check whether a "field CMP const" comparison uses a constant 
  that's longer than the field it is compared to.  Make this check only for string columns,
  also compare character lengths, not byte lengths.

allocate thd_proc_info string in thd memroot, not on the stack, so that it
won't be overwritten while another thread might be printing it

sort Com_ counters. No simple fix for Binlog_ variables.

MDEV-5153: Server crashes in Item_ref::fix_fields on 2nd execution of PS with LEFT JOIN and MERGE view or SELECT SQ

1. Transformation of row IN subquery made the same as single value.
2. replace_where_subcondition() made working on several layers of OR/AND because it called on expression before fix_fields().

(and valgrind warnings)

* move thd userstat initialization to the same function
  that was adding thd userstat to global counters.
* initialize thd->start_bytes_received in THD::init
  (when thd->userstat_running is set)

Singular Item_equal support added.

The problem was that during constant table substitution Item_equal become containing only one constant which was not supported internally.

mysql_upgrade should do --skip-write-binlog by default

by convention query execution time should be 0 if its start_time is 0
(this was lost when fixing MDEV-4578)

port mysqld_safe numa extensions from percona-server: --flush-caches and --numa-interleave

1. use an appropriate errno code
2. put a comma between the errno and the error message text

before running mysql_install_db check for the existence
of $datadir/mysql, not simply $datadir ($datadir might be mounted on
a separate device - exists, but empty)