msdosfs manuals: Improve visibility and linking

+ add (FAT) to all descriptions to enable apropos fat
+ xref all msdosfs(4) utilities in base to msdosfs(4)
+ xref msdosfs(4) to all msdosfs(4) utilities
+ remove unrelated fsck_ffs(8) from fsck_msdos(8)

MFC after: 3 days
Reviewed by: mhorne
Approved by: mhorne (mentor)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1557