Setting a key_cache_block_size which is not a power of 2
could corrupt MyISAM tables.

A couple of computations in the key cache code use bit
operations which do only work if key_cache_block_size
is a power of 2.

Replaced bit operations by arithmetic operations
to make key cache able to handle block sizes that are
not a power of 2.


include/keycache.h:
  Bug#28478 - Improper key_cache_block_size corrupts MyISAM tables
  Removed element 'key_cache_shift' from KEY_CACHE after
  the changes in mf_keycache.c made it unused.
mysql-test/r/key_cache.result:
  Bug#28478 - Improper key_cache_block_size corrupts MyISAM tables
  Added test result
mysql-test/t/key_cache.test:
  Bug#28478 - Improper key_cache_block_size corrupts MyISAM tables
  Added test
mysys/mf_keycache.c:
  Bug#28478 - Improper key_cache_block_size corrupts MyISAM tables
  Replaced bit operations by arithmetic operations
  to make key cache able to handle block sizes that are
  not a power of 2.