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
# https://editorconfig.org/
root = true
[*]
charset = utf-8
end_of_line = lf
indent_style = tab
insert_final_newline = true
trim_trailing_whitespace = true
max_line_length = 150
[CHANGES]
indent_size = 4
indent_style = space
[configure]
indent_size = 3
indent_style = space
[*.xml]
# xmllint --nsclean --encode UTF-8 --noent --format -
indent_size = 2
indent_style = space