lxcapi_restore shouldn't steal the calling process

lxcapi_restore shouldn't steal the calling process

Previously, lxcapi_restore used the calling process as the lxc monitor process
(and just never returned), requiring users to fork before calling it. This, of
course, would cause problems for things like LXD, which can't fork.

Now, restore() forks the monitor as a child of the process that calls it. Users
who want to daemonize the restore process need to fork themselves.
lxc-checkpoint has been updated to reflect this behavior change.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>