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