plugins: hook-scripts: Add

Add a plugin that will allow the user to trigger the execution of
scripts from plugin hook points. This is particularly useful for early
boot debugging that needs to take place before regular services are
available.

For example, let's say that you want to enable some kernel tracing
before modules are loaded. With hook-scripts, you can just drop in a
shell script in /libexec/finit/hook/mount/all/ that will poke the
right control files in tracefs.

doc/plugins.md

@@ -34,6 +34,12 @@ For your convenience a set of *optional* plugins are available:
 * *dbus.so*: Setup and start system message bus, D-Bus, at boot.
   _Optional plugin._
 
+* *hook-scripts.so*: Trigger the execution of scripts from plugin hook
+  points (see [Hooks](#hooks)).  Scripts are located in
+  `/libexec/finit/hook` by default, this can be build-time customized
+  by using the `--with-hook-scripts-path=PATH` argument to
+  `configure`.  _Optional plugin._
+
 * *hotplug.so*: Setup and start either udev or mdev hotplug daemon, if
   available.  Enabled by default.