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.