API support for container snapshots (v2)

I'll write a 'lxc-snapshot' program using this api next, but here is
a first shot at the actual API functionality.

See the comments in lxccontainer.h and the example usage in
src/tests/snapshot.c for details.

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>
introduce lxc-snapshot

Hopefully someone else will come in and spruce it up :)

Some 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>