If we are euid==0 or XDG_RUNTIME_DIR is not set, then use
/run/lock/lxc/$lxcpath/$lxcname as before.  Otherwise,
use $XDG_RUNTIME_DIR/lock/lxc/$lxcpath/$lxcname.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: Stéphane Graber <stephane.graber@canonical.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>