Add apcupsd 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 50 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";

Bug/annoyance 1: When apcupsd calls "wall" (on powerfail etc. events),
it prints an error message because stdout is not connected to a tty (it
is connected to the journal):

  wall: cannot get tty name: Inappropriate ioctl for device

The message still gets through though, to ctrl-alt-f[1-6] terminals.

Bug/annoyance 2: apcupsd tries to call "mail" (on powerfail etc.
events), and that fails because I'm not passing in any mail program at
the moment (because that would require more configuration options). A
solution to this would be to simply let the user fully configure the
apcupsd event handling logic in nix.
Merge pull request #204 from bjornfor/apcupsd-service

Add apcupsd service
Add uptime module

Signed-off-by: Shea Levy <shea@shealevy.com>
Only include /bin/sh (not all of /bin) in the chroot

Signed-off-by: Shea Levy <shea@shealevy.com>
Allow specifying packages whose closures should be in the chroot

Signed-off-by: Shea Levy <shea@shealevy.com>
Documentation improvement

Signed-off-by: Shea Levy <shea@shealevy.com>
Mount /var and some other filesystems automatically in stage 1

Bad things happen if /var is mounted in a late stage.
Set permissions on /var/log/journal properly

This makes the system journal readable by users in the
systemd-journal, wheel and adm groups.  It also allows users to read
their own journals.

Note that this doesn't change the permissions of existing journals.