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

[asomers]

Summary

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.

Test Plan

1. Create a ZFS or UFS filesystem with mountpoint set to /usr/home
2. Add an NFS entry for /usr/home to /etc/fstab. Don't mount it.
3. Run "mount -d -a -t nfs". It should print the entry from step 2
4. Mount /usr/home
5. Run "mount -d -a -t nfs". It should print nothing.

[asomers]

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