Add support for checkpoint and restore via CRIU

This patch adds support for checkpointing and restoring containers via CRIU.
It adds two api calls, ->checkpoint and ->restore, which are wrappers around
the CRIU CLI. CRIU has an RPC API, but reasons for preferring exec() are
discussed in [1].

To checkpoint, users specify a directory to dump the container metadata (CRIU
dump files, plus some additional information about veth pairs and which
bridges they are attached to) into this directory. On restore, this
information is read out of the directory, a CRIU command line is constructed,
and CRIU is exec()d. CRIU uses the lxc-restore-net callback (which in turn
inspects the image directory with the NIC data) to properly restore the
network.

This will only work with the current git master of CRIU; anything as of
a152c843 should work. There is a known bug where containers which have been
restored cannot be checkpointed [2].

[1]: http://lists.openvz.org/pipermail/criu/2014-July/015117.html
[2]: http://lists.openvz.org/pipermail/criu/2014-August/015876.html

v2: fixed some problems with the s/int/bool return code form api function
v3: added a testcase, fixed up the man page synopsis
v4: fix a small typo in lxc-test-checkpoint-restore
v5: remove a reference to the old CRIU_PATH, and a bad error about the same

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
handle hashed command socket names (v2)

With the new hashed command socket names (e85898415c), it's possible to
have something like below;

[caglar@qop:~/go/src/github.com/lxc/go-lxc(master)] cat /proc/net/unix | grep lxc
0000000000000000: 00000002 00000000 00010000 0001 01 53465 @lxc/d086e835c86f4b8d/command
[...]

list_active_containers reads /proc/net/unix to find all running
containers but this new format no longer includes the container name or
its lxcpath.

This patch introduces two new commands (LXC_CMD_GET_NAME and
LXC_CMD_GET_LXCPATH) and starts to use those in list_active_containers
call.

changes since v1:
 - added sanity check proposed by Serge

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Fix build failure due to wrong test name

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Add lxc-restore-net to extra_DIST

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>