don't fail lxc-init if we couldn't mount proc

In general proc gets mounted ahead of time, so init shouldn't
have to do it.  Without this patch, you cannot

	lxc-execute -n x1 -s lxc.cap.drop=sys_admin /bin/bash

(See https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1253669 for
a bug about this)

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