makefs: add msdosfs (FAT) support

Add FAT support to makefs by copying some files from sys/fs/msdosfs/ and
updating others with changes from NetBSD.

The six files copied from sys/fs/msdosfs at r348251 and modified are:
denode.h direntry.h fat.h msdosfs_fat.c msdosfs_lookup.c msdosfsmount.h

I would prefer to avoid the duplication, but reluctance to doing so was
expressed in a previous review (D11197); for now copy the files and
revisit in the future.

Submitted by: Siva Mahadevan
Discussed with: cem, imp
MFC after: 1 month
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16438