util.startup: Support systemd Type=notify service type
This lets Prosody report its lifecycle status to systemd, so it knows
when Prosody has completed its startup, when it's reloading and shutting
down.
Both Type=notify and Type=notify-reload is supported
Example systemd .service configuration snippet:
[Service]
Type=notify
all: prosodyctl.man
%.man: %.markdown
pandoc -s -t man -o $@ $^