on packed MyISAM tables

When fixing the indexes with "myisamchk -rq" after compressing
the table with "myisampack", an optionally supplied extension
".MYI" of the index file was not detected. The extension was
appended unconditionally. The result was ".MYI.MYI".

Now an extension is no longer appended if present already.
Thanks to David Shrewsbury for providing this patch.

Another problem was a misplaced parenthesis. We did never unpack
the file name ("~/..") and always returned a real path.

No test case. This is manually tested with the utilities
"myisampack" and "myisamchk".


storage/myisam/mi_create.c:
  Bug#26782 - Patch: myisamchk -rq creates .MYI.MYI file
              on packed MyISAM tables
  Added code to detect existing extension on index file name.
  Thanks to David Shrewsbury for providing this patch.
  Additionally fixed an parenthesis. We did never unpack
  the file name and always returned real path.