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.

plugins/Makefile.am

@@ -64,6 +64,10 @@ if BUILD_DBUS_PLUGIN
 pkglib_LTLIBRARIES += dbus.la
 endif
 
+if BUILD_HOOK_SCRIPTS_PLUGIN
+pkglib_LTLIBRARIES += hook-scripts.la
+endif
+
 if BUILD_HOTPLUG_PLUGIN
 pkglib_LTLIBRARIES += hotplug.la
 endif