Mount msdosfs with longnames support by default.

The old behavior depended on the FAT version and on what files were in the
root directory. "mount_msdosfs -o shortnames" is still supported.

Reviewed by: wblock, cem
Discussed with: trasz, adrian, imp
MFC after: 4 weeks
X-MFC-Notes: Don't MFC the removal of findwin95
Differential Revision: https://reviews.freebsd.org/D8018