parallel read access to IO_CACHE


include/my_sys.h:
  parallel read access to IO_CACHE
include/myisam.h:
  multithreaded repair-by-sort code
myisam/mi_cache.c:
  parallel read access to IO_CACHE
  misc cleanups
myisam/mi_check.c:
  multithreaded repair-by-sort code
myisam/myisamchk.c:
  multithreaded repair-by-sort code
myisam/myisamdef.h:
  multithreaded repair-by-sort code
myisam/sort.c:
  multithreaded repair-by-sort code
mysys/mf_iocache.c:
  parallel read access to IO_CACHE