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)

Switch the default WARNS level for sbin/ to 6.

Submitted by: Ulrich Spörlein

- Move from mount(2) to nmount(2). This should allow to convert MNT_SNAPSHOT
flag from a mount flag to FS-specific flag.
- Simplify usage. Instead of 'mksnap_ffs /mnt/foo /mnt/foo/snap' allow to
give only one argument: 'mksnap_ffs /mnt/foo/snap'. Old usage is also
accepted for now.
- Add an example of how to mount a snapshot.

Add the mksnap_ffs command to the sbin directory. This setuid root
program allows users in the operator group to take filesystem
snapshots. Its first use will be in support of `dump -L'.

Approved by: Technical Review Board <trb@FreeBSD.org>
Sponsored by: DARPA & NAI Labs.