Extend condition handling with support for simple config files

[troglobit]

By design, the condition system in Finit currently does not support checking for plain files. The reason for this was mainly to avoid starting with the wrong version of a daemon config file. However, on simpler systems with more static setups, e.g., generic Linux distributions, it is common practise to look for the existence of /etc/daemon.conf before attempting to start a service.

To work around this "wrapper scripts" were invented, which also allow for sourcing files in /etc/default/demon and checking DAEMON_ENABLED environment variable and similar.

[troglobit]

Condition syntax proposal: <file/path/to/file>. For each ready service a new plugin has an inotify watcher for /path/to, which must be an absolute path.

[troglobit]

This was dropped from the 4.0 milestone because of its inherent complexity. Unlike the pidfile plugin, which has a root for the files it monitors, the file plugin would have no base but have to closely follow run/task/services that declare a <file/*> condition.

For many use-cases the new env: support will hopefully be sufficient. Anyone else is of course welcome to pick this one up and submit a pull-request! Just make sure to discuss the design (here) with us first :-)

[troglobit]

For a deeper discussion and a possible way forward, see #185