newfs_msdos: rework option parsing to match NetBSD

NetBSD split newfs_msdos in two so that they can reuse the file system
creation part in makefs. This change is a step on the path of bringing
that support to FreeBSD.

Reviewed by: kib, pfg
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3905