commands: handle epipe

If we start a lxc_wait on a container while it is exiting, it is
possible that we open the command socket, then the command socket
monitor closes all its mainloop sockets and exit, then we send our
credentials. Then we get killed by SIGPIPE.

Handle that case, recognizing that if we get sigpipe then the
container is (now) stopped.

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