Give the user a warning if he tries to destroy the default key cache.


mysql-test/r/key_cache.result:
  Bug#10473 - Can't set 'key_buffer_size' system variable to ZERO
  The test result.
mysql-test/t/key_cache.test:
  Bug#10473 - Can't set 'key_buffer_size' system variable to ZERO
  The test case.
sql/share/errmsg.txt:
  Bug#10473 - Can't set 'key_buffer_size' system variable to ZERO
  The new message string.