This change fixes test failure for partition_repair_myisam
and partition_recover_myisam.

The problem was that:
- the file std_data/corrupt_crash.MYI
  contains charsetnr=63 (binary) for index segments.
- the new .frm file contains charsetnr=8 (latin1) for
  the same segments.
As a result REPAIR refused to repair frm+MYI files
with different segment definition.

This fix restores the old behavior for frm and MYI files:
they now store charsetnr=63 for numeric/datetime data types.