Overhaul plugins and plugin architecture

[troglobit]

As discussed in #429, calling dlclose() at shutdown just to trigger the plugin exit code to do cleanup, or other tasks is risky.

There's also a few plugins that have become foundational and are more core functionality in Finit today, so the term "plugin" for them is quite misleading. A better term for this type of functionality could be extensions.

This issue proposes:

1. Drop support for PLUGIN_EXIT(), i.e., drop dlclose()
2. Relocate foundational plugins to src/, e.g., pidfile.so, bootmisc.so, etc.
3. Possibly move optional plugins to https://github.com/troglobit/finit-plugins repo