Introduced a new free blocks list. Free blocks are now re-used before
new blocks are allocated from the pool. There is a new status variable 
which can be queried by "show status like key_blocks_unused".


include/keycache.h:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  free_block_list is the new free blocks list. It is implemented like a stack (LIFO).
  blocks_unused holds the number of never used blocks plus the number of blocks in the free list.
  Removed the variable global_blocks_used, as it was always the same as blocks_used.
mysql-test/r/key_cache.result:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  Inserted some commands which show how key_blocks_used and key_blocks_unused work.
mysql-test/t/key_cache.test:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  Inserted some commands which show how key_blocks_used and key_blocks_unused work.
mysys/mf_keycache.c:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  Introduced a new free blocks list. The introductory comment says it all (I hope).
  Removed the variable global_blocks_used, as it was always the same as blocks_used.
sql/mysqld.cc:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  The blocks_unused count can be queried by "show status like key_blocks_unused".
  Removed the variable global_blocks_used, as it was always the same as blocks_used.
  Introduced SHOW_KEY_CACHE_CONST_LONG for status variables that
  must not be modified (i.e. flushed to zero).
sql/sql_show.cc:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  Introduced SHOW_KEY_CACHE_CONST_LONG for status variables that
  must not be modified (i.e. flushed to zero).
sql/sql_test.cc:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  Removed the variable global_blocks_used, as it was always the same as blocks_used.
sql/structs.h:
  WL#1700 - Properly count key_blocks_used and key_blocks_current.
  Introduced SHOW_KEY_CACHE_CONST_LONG for status variables that
  must not be modified (i.e. flushed to zero).