Allow multiple indexes to be dropped. (Bug #53256)

Bug#53417 my_getwd() makes assumptions on the buffer sizes which not always hold true
      
The mysys library contains many functions for rewriting file paths. Most of these
functions makes implicit assumptions on the buffer sizes they write to. If a path is put
in my_realpath() it will propagate to my_getwd() which assumes that the buffer holding
the path name is greater than 2. This is not true in cases.
      
In the special case where a VARBIN_ITEM is passed as argument to the LOAD_FILE function
this can lead to a crash.
      
This patch fixes the issue by introduce more safe guards agaist buffer overruns.

are different in 5.1 comparing to that of 5.5, so a hand port is
necessary to avoid wrong default option to be set by a simple
branch merge.

This is the 5.1 merge and extension of the fix.
The server was happily accepting paths in table name in all places a table
name is accepted (e.g. a SELECT). This allowed all users that have some 
privilege over some database to read all tables in all databases in all
mysql server instances that the server file system has access to.
Fixed by :
1. making sure no path elements are allowed in quoted table name when
constructing the path (note that the path symbols are still valid in table names
when they're properly escaped by the server).
2. checking the #mysql50# prefixed names the same way they're checked for
path elements in mysql-5.0.

buf_flush_insert_into_flush_list(),
buf_flush_insert_sorted_into_flush_list(),
buf_flush_post_to_doublewrite_buf(): Check that the page is initialized.

buf_flush_insert_into_flush_list(),
buf_flush_insert_sorted_into_flush_list(),
buf_flush_post_to_doublewrite_buf(): Check that the page is initialized.

would get the block, new_block in the wrong order.
Fixing that would have complicated the function even further
for this marginal case.

      
When issuing a 'SET GLOBAL SQL_SLAVE_SKIP_COUNTER' statement, the previous
position along with the new position is dumped into the error log. Namely,
the following information is printed out: skip_counter, group_relay_log_name
and group_relay_log_pos.

When split_rec==NULL, choose the correct node pointer key (first_rec).

      
When issuing a 'CHANGE MASTER TO' statement, key elements of the previous
state, namely the host, port, the master_log_file and the master_log_pos
are dumped into the error log.

Iterative patch improvement. Previously committed patch
caused wrong result on Windows. The previous patch also
broke secure_file_priv for symlinks since not all file
paths which must be compared against this variable are
normalized using the same norm.

The server variable opt_secure_file_priv wasn't
normalized properly and caused the operations
LOAD DATA INFILE .. INTO TABLE ..
and
SELECT load_file(..)
to do different interpretations of the 
--secure-file-priv option.
     
The patch moves code to the server initialization
routines so that the path always is normalized
once and only once.
      
It was also intended that setting the option
to an empty string should be equal to 
lifting all previously set restrictions. This
is also fixed by this patch.


mysql-test/r/loaddata.result:
  * Removed test code which will currently break the much used --mem feature of mtr.
mysql-test/t/loaddata.test:
  * Removed test code which will currently break the much used --mem feature of mtr.
sql/item_strfunc.cc:
  * Replaced string comparing code on opt_secure_file_priv with an interface which guarantees that both file paths are normalized using the same norm on all platforms.
sql/mysql_priv.h:
  * Added signature for is_secure_file_path()
sql/mysqld.cc:
  * New function for checking if a path compatible with the secure path restriction.
  * Added initialization of the opt_secure_file_priv variable.
sql/sql_class.cc:
  * Replaced string comparing code on opt_secure_file_priv with an interface which guarantees that both file paths are normalized using the same norm on all platforms.
sql/sql_load.cc:
  * Replaced string comparing code on opt_secure_file_priv with an interface which guarantees that both file paths are normalized using the same norm on all platforms.

The server was not checking the supplied to COM_FIELD_LIST table name
for validity and compliance to acceptable table names standards.
Fixed by checking the table name for compliance similar to how it's 
normally checked by the parser and returning an error message if 
it's not compliant.

buf_page_get_gen(): Assert that buf_zip_decompress() succeeds.
Callers are not prepared for a NULL return value. (Bug #53248)