rc: virtual_oss: Wait for process to exit

Sometimes virtual_oss processes do not exit immediatelly. If we do not
wait for the processes to fully exit before returning from
virtual_oss_stop(), then the service restart operation might call
virtual_oss_start() too early and fail, because it will think the
service wasn't stopped and is still running.

Reported by:	jrm
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	0mp, jrm
Pull-Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/33

libexec/rc/rc.d/virtual_oss

@@ -76,8 +76,10 @@ stop_instance()
 	if [ -z "${instance_args}" ]; then
 		warn "no such config: ${config}"
 	else
+		pid="$(cat "${pidpath}/${config}.pid")"
 		startmsg -n "Stopping virtual_oss config: ${config}: "
-		kill "$(cat "${pidpath}/${config}.pid")"
+		kill "${pid}"
+		pwait "${pid}"
 		rm -f "${pidpath}/${config}.pid"
 		startmsg "done"
 	fi