fsck_msdosfs(8): Introduce -B option as no-op

When performing a background fsck on msdosfs devices, it ends
with the following error:

fsck_msdosfs: illegal option -- B
usage: fsck_msdosfs -p [-f] filesystem ...

fsck_msdosfs [-ny] filesystem ...

Introduce -B option as a compatibility with fsck_ffs(8) and
also update the descriptions for -B and -C in fsck_msdosfs(8)
manual page.

Reviewed by: imp
Approved by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48132