ovs-lib.in: Cleanup old socket and pidfiles in stop_daemon

ovs-lib.in: Cleanup old socket and pidfiles in stop_daemon

Currently if a client crashes (signal 11) the unix socket (.ctl) and the
pidfile may not be deleted when you use ovs-ctl stop or restart.

Moreover since ovs-appctl is used on a closed socket some warnings are
printed.

This commit deletes the pidfile and the unix socket then returns without
running ovs-appctl if the pidfile point to a not-existing pid.

Reported-at: https://bugzilla.redhat.com/1667845
Reported-by: Candido Campos <ccamposr@redhat.com>
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>