s6-notify: support fd-based s6 notification protocol (#1466)

Dinit and s6 supervision suite are known to support this protocol.

See https://skarnet.org/software/s6/notifywhenup.html and
https://davmac.org/projects/dinit/man-pages-html/dinit-service.5.html#ready

Signed-off-by: Mobin Aydinfar <mobin@mobintestserver.ir>