Add acpupsd service

apcupsd is a daemon for controlling APC UPSes. It is very simple to
configure. If you have an USB based UPS, the default settings should be
useable without further adjustments:

  services.apcupsd.enable = true;

This will give you autodetection of USB UPSes, network access limited to
localhost (for security) and the shutdown sequence will be started when
the system when the battery level is below 20 percent, or when the UPS
has calculated that it has 5 minutes or less of remaining power-on time.

You can provide your own configuration file contents with this option:

  services.apcupsd.configText = "contents of apcupsd.conf";