sh/s6.sh: update s6-svc syntax to be valid for 2.2.0.0+
sh/s6.sh: update s6-svc syntax to be valid for 2.2.0.0+

The s6-svc syntax changed for wait-up, wait-ready, wait-down, and
wait-finished. This changes the s6 handling script to use the current
valid syntax.

This fixes #65.

williamh/openrc,OpenRC/openrc,williamh/openrc,dwfreed/openrc,dwfreed/openrc,OpenRC/openrc,dwfreed/openrc,williamh/openrc,OpenRC/openrc