switch-to-configuration.pl is currently hard-coded to assume that if a
unit is in the "auto-restart" state that something has gone wrong, but
this is not strictly true. For example, I run offlineimap as a oneshot
service restarting itself every minute (on success). NixOS currently
thinks that offlineimap has failed to start as it enters the
auto-restart state, because it doesn't consider why the unit failed.
This commit changes switch-to-configuration.pl to inspect the full
status of a unit in auto-restart state, and now only considers it failed
if the ExecMainStatus is non-zero.
Currently switch-to-configuration.pl uses system() calls to interact
with DBus. This can be error prone, especially when we are parsing
output that could change. In this commit, almost all calls to the
systemctl binary have been replaced with equivalent operations via DBus.
