Fix "mount -a" for NFS and ZFS filesystems with shared mountpoints

Fix "mount -a" for NFS and ZFS filesystems with shared mountpoints

sbin/mount.c
	Check whether an fstab entry has the same fstype as a mounted
	filesystem before declaring it to be mounted. This will allow NFS
	filesystems that share a mountpoint with a local filesystem to be
	automatically mounted at boot.

	This is not such an unusual situation. For example, if somebody uses
	the standard installer with a ZFS root, he'll get a /usr/home
	filesystem, even though he may choose to mount /usr/home over NFS.

Reviewed by:	trasz
MFC after:	4 weeks
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D4556