makefs: add msdosfs (FAT) support

MFC r351232: makefs.8: expand description of image size

Submitted by: ryan_freqlabs.com, Siva Mahadevan

MFC r351273: 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

MFC r351275: makefs: avoid "dereferencing 'void *' pointer" warnings

On GCC 4.2.1 archs

MFC r351302: makefs.8: update history

• ported to FreeBSD and first appeared in 8.0
• Christos Zoulas added the FAT support that I imported

MFC r351319: makefs: use char * not void * for buf b_data, drop casts in msdos

(The kernel uses caddr_t.)

Suggested by: cem

MFC r351415 (delphij): Properly update FSInfo block after generation.

After populating the filesystem, write a FSInfo block with
proper information.

Sponsored by: The FreeBSD Foundation