sbin/newfs_msdos: Bring in -A/-T options from FreeBSD

Basically from FreeBSD/Git 284893ba and 8255b708.

sys/vfs/msdosfs: Whitespace cleanups

Mostly trailing whitespace and >80 chars per line.
Also remove redundant static function prototypes.

newfs_msdos(8): Sync with FreeBSD.

* New options: '-C size' to create an empty image of the specified
size and '-@ offset' to add the image at the specfied offset.

* Separate some parts into mk

newfs_msdos(8): Sync with FreeBSD.

* New options: '-C size' to create an empty image of the specified
size and '-@ offset' to add the image at the specfied offset.

* Separate some parts into mkfs_msdos.c for later perusal by
makefs(8), which we have yet to bring in.

* Numerous improvements and bug fixes.

* Raise WARNS to 6.

Taken-from-and-thanks-to: FreeBSD and NetBSD

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