API support for container snapshots (v2)

The api allows for creating, listing, and restoring of container
snapshots.  Snapshots are created as snapshot clones of the
original container - i.e. btrfs and lvm will be done as snapshot,
a directory-backed container will have overlayfs snapshots.  A
restore is a copy-clone, using the same backing store as the
original container had.

Changelog:

 . remove lxcapi_snap_open, which wasn't defined anyway.
 . rename get_comment to get_commentpath
 . if no newname is specified at restore, use c->name (as we meant to)
   rather than segving.
 . when choosing a snapshot index, use the correct path to check for.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
introduce lxc-snapshot

Hopefully someone else will come in and spruce it up :)  This
version is as simple as can be

lxc-snapshot -n a1
	create a snapshot of a1
echo "second commit" > /tmp/a
lxc-snapshot -n a1 -c /tmp/a
	create a snapshot of a1 with /tmp/a as a commit comment
lxc-snapshot -n a1 -L
	list a1's snapshots
lxc-snapshot -n a1 -L -C
	list a1's snapshots along with commit comments
lxc-snapshot -n a1 -r snap0 a2
	restore snapshot 0 of a1 as container a2

Some easy nice-to-haves:

1. sort snapshots in the list
2. allow a comment to be given in-line
3. an option to remove a snapshot?

Removing a snapshot can just as well be done with

	lxc-destroy -P /var/lib/lxcsnaps/c1 -n snap2

so I leave it to others to decide whether they really want
it, and provide the patch if so.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
error.c: don't return error if container init signaled

We log that at INFO level in case it is needed.  However, in a modern
kernel a container which was shut down using 'shutdown' will always
have been signaled with SIGINT.  Making lxc-start return an error to
reflect that seems overkill.

It's *conceivable* that someone is depending on this behavior, so I'm
sending this out for anyone to NACK, but if I hear no complaints I'll
apply.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>