Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

mdoc: consistently spell our email addresses <foo@FreeBSD.org>

Reviewed by: ru

Mark ftwv as __unused. This compare function does not need it.

Imagine a situation where:

# ls -ld /mnt/{foo,bar}
drwxr-xr-x 3 root wheel 512 Mar 16 06:56 /mnt/bar
lrwxr-xr-x 1 root wheel 3 Mar 16 12:10 /mnt/foo -> bar

# grep /mnt/foo /etc/fstab
/dev/da1 /mnt/foo ufs rw 0 0

Which means, we give symbolic link as a mount point to mount(8), but mount(8)
use realpath(3) before mounting the file systems, so we get:

# mount | grep /dev/da1
/dev/da1 on /mnt/bar (ufs, local)

Before the commit:

# snapinfo /mnt/foo
usage: snapinfo [-v] -a
snapinfo [-v] mountpoint
# snapinfo /mnt/bar
/mnt/bar/snap

This commit makes snapinfo(8) to first realpath(3) the given mount point and
now we have:

# snapinfo /mnt/foo
/mnt/bar/snap
# snapinfo /mnt/bar
/mnt/bar/snap

Pass special device to the ufs_disk_fillout() function, instead of mount
point path. This way we properly handle the case when file system listed
in /etc/fstab was unmounted and another file system was mounted on the
same mount point.

Fix version number when the feature was added.

Spotted by: ceri
MFC after: 1 day

snapinfo -- show snapshot location on UFS file systems

Glanced over by: kan
Review by: ru (snapshot.8)
MFC: TBD