msdosfs: use mem{cpy,move,set} instead of bcopy,bzero

This somewhat simplifies use of msdosfs code in userland (for makefs),
reduces diffs with NetBSD and is standard C as of C89.

Reviewed by: imp
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11014