USING MYISAM_USE_MMAP ON WINDOWS

When OPTIMIZE/REPAIR TABLE is switching to new data file,
old data file is removed while memory mapping is still
active.

With 5.1 implementation of nt_share_delete() it is not
permitted to remove mmaped file.

This fix disables memory mapping for mi_repair() operations.

mysql-test/r/myisam.result:
  A test case for BUG#11757032.
mysql-test/t/myisam.test:
  A test case for BUG#11757032.
storage/myisam/ha_myisam.cc:
  mi_repair*() functions family use file I/O even if memory
  mapping is available.
  
  Since mixing mmap I/O and file I/O may cause various artifacts,
  memory mapping must be disabled.
storage/myisam/mi_delete_all.c:
  Clean-up: do not attempt to remap file after truncate, since
  there is nothing to map.