Command line and configuration file option 'key_cache_block_size'
was reduced by MALLOC_OVERHEAD (8 in a production server, 36 in a
debug server) from the user supplied value and restricted it to
the greatest multiple of 512 less or equal to the reduced value.

This patch changes option 'key_cache_block_size' to not deduce
MALLOC_OVERHEAD from the input value. However, the restriction
to a multiple of 512 is still done.