pointed data.
I ran gcc -Wcast-qual on storage/maria, this identified un-needed casts,
a couple of functions which said they had a const parameter though
they changed the pointed content! This is fixed here. Some suspicious
places receive a comment.
The original intention of running -Wcast-qual was to find what code
changes R-tree keys: I added const words, but hidden casts
like those of int2store (casts target to (uint16*)) removed const
checking; -Wcast-qual helped find those hidden casts.
Log handler does not change the content pointed by LEX_STRING::str it
receives, so we now use a struct which has a const inside, to emphasize
this and be able to pass "const uchar*" buffers to log handler
without fear of their content being changed by it.
One-line fix for a merge glitch (when merging from MyISAM).


include/m_string.h:
  As Maria's log handler uses LEX_STRING but never changes the content
  pointed by LEX_STRING::str, and assigns uchar* into this member most
  of the time, we introduce a new struct LEX_CUSTRING
  (C const U unsigned) for the log handler.
include/my_global.h:
  In macros which read pointed content: use const pointers so that
  gcc -Wcast-qual does not warn about casting a const pointer to non-const.
include/my_handler.h:
  In macros which read pointed content: use const pointers so that
  gcc -Wcast-qual does not warn about casting a const pointer to non-const.
  ha_find_null() does not change *a.
include/my_sys.h:
  insert_dynamic() does not change *element.
include/myisampack.h:
  In macros which read pointed content: use const pointers so that
  gcc -Wcast-qual does not warn about casting a const pointer to non-const.
mysys/array.c:
  insert_dynamic() does not change *element
mysys/my_handler.c:
  ha_find_null() does not change *a
storage/maria/ma_bitmap.c:
  Log handler receives const strings now
storage/maria/ma_blockrec.c:
  Log handler receives const strings now.
  _ma_apply_undo_row_delete/update() do change *header.
storage/maria/ma_blockrec.h:
  correct prototype
storage/maria/ma_check.c:
  Log handler receives const strings now. Un-needed casts
storage/maria/ma_checkpoint.c:
  Log handler receives const strings now
storage/maria/ma_checksum.c:
  unneeded cast
storage/maria/ma_commit.c:
  Log handler receives const strings now
storage/maria/ma_create.c:
  Log handler receives const strings now
storage/maria/ma_dbug.c:
  fixing warning of gcc -Wcast-qual
storage/maria/ma_delete.c:
  Log handler receives const strings now
storage/maria/ma_delete_all.c:
  Log handler receives const strings now
storage/maria/ma_delete_table.c:
  Log handler receives const strings now
storage/maria/ma_dynrec.c:
  fixing some warnings of gcc -Wcast-qual. Unneeded casts removed.
  Comment about function which lies.
storage/maria/ma_ft_parser.c:
  fix for warnings of gcc -Wcast-qual, removing unneeded casts
storage/maria/ma_ft_update.c:
  less casts, comment
storage/maria/ma_key.c:
  less casts, stay const (warnings of gcc -Wcast-qual)
storage/maria/ma_key_recover.c:
  Log handler receives const strings now
storage/maria/ma_loghandler.c:
  Log handler receives const strings now
storage/maria/ma_loghandler.h:
  Log handler receives const strings now
storage/maria/ma_loghandler_lsn.h:
  In macros which read pointed content: use const pointers so that
  gcc -Wcast-qual does not warn about casting a const pointer to non-const.
storage/maria/ma_page.c:
  Log handler receives const strings now; more const
storage/maria/ma_recovery.c:
  Log handler receives const strings now
storage/maria/ma_rename.c:
  Log handler receives const strings now
storage/maria/ma_rt_index.c:
  more const, to emphasize that functions don't change pointed content.
  best_key= NULL was forgotten during merge from MyISAM a few days ago,
  was causing a Valgrind warning
storage/maria/ma_rt_index.h:
  new proto
storage/maria/ma_rt_key.c:
  more const
storage/maria/ma_rt_key.h:
  new proto
storage/maria/ma_rt_mbr.c:
  more const for functions which deserve it
storage/maria/ma_rt_mbr.h:
  new prototype
storage/maria/ma_rt_split.c:
  make const what is not changed.
storage/maria/ma_search.c:
  un-needed casts, more const
storage/maria/ma_sp_key.c:
  more const
storage/maria/ma_unique.c:
  un-needed casts.
storage/maria/ma_write.c:
  Log handler receives const strings now
storage/maria/maria_def.h:
  some more const
storage/maria/unittest/ma_test_loghandler-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_first_lsn-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_max_lsn-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_multigroup-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_multithread-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_noflush-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_nologs-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_pagecache-t.c:
  Log handler receives const strings now
storage/maria/unittest/ma_test_loghandler_purge-t.c:
  Log handler receives const strings now