myisam tables

Queries following TRUNCATE of partitioned MyISAM table
may crash server if myisam_use_mmap is true.

Internally this is MyISAM bug, but limited to partitioned
tables, because MyISAM doesn't use ::delete_all_rows()
method for TRUNCATE, but goes via table recreate instead.

MyISAM didn't properly fall back to non-mmaped I/O after
mmap() failure. Was not repeatable on linux before, likely
because (quote from man mmap):
  SUSv3  specifies  that  mmap() should fail if length is 0.
  However, in kernels before 2.6.12, mmap() succeeded in
  this case: no mapping was created and the call returned
  addr. Since kernel 2.6.12, mmap() fails with the error
  EINVAL for this case.

mysql-test/r/partition.result:
  A test case for BUG#51868.
mysql-test/t/partition.test:
  A test case for BUG#51868.
storage/myisam/mi_delete_all.c:
  _mi_unmap_file() is compressed record format specific,
  which is read-only. As compressed MyISAM data files are
  read-only, we must never use _mi_unmap_file() in
  mi_delete_all_rows().
storage/myisam/mi_dynrec.c:
  Make myisam mmap code more durable to errors:
  - set file_read/file_write handlers if mmap succeeded;
  - reset file_read/file_write handlers on unmap.
storage/myisam/mi_extra.c:
  Moved file_read/file_write handlers initialization to
  mi_dynmap_file().
storage/myisam/myisamdef.h:
  Added mi_munmap_file() declaration.