plugins/modules-load: change default, load modules in runlevel S

Instead of waiting until runlevel 2, 3, 4, or 5 to load modules with
this plugin, we should load modules as soon as possible, i.e., in
runlevel S.  This also means that Finit will clean up all the tasks
so we don't have to see them in `initctl status`.

To allow users to revert to the old behavior, we add a new setting

    set runlevel 2345

that per modules-load.d/file changes the runlevel for all modprobe
commands.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/plugins.md

@@ -64,10 +64,14 @@ For your convenience a set of *optional* plugins are available:
   name of the module to load.  Any line starting with the standard UNIX
   comment character, `#`, or `;`, is skipped.
   
-  Modules are loaded when entering runlevel `[2345]` using the `task`
-  stanza.  Each module gets a unique `name:modprobe.foo`, and
-  optional`:ID`.  The `:ID` is a globally incremented index, which can
-  be disabled per file (anywhere) using the following config line:
+  Modules are by default loaded in runlevel `S` using the `task` stanza.
+  Each module gets a unique `name:modprobe.foo`, and optional`:ID`.  The
+  runlevel can be changed per file using:
+
+        set runlevel 2345
+
+  The `:ID` is a globally incremented index, which can be disabled per
+  file (anywhere) using the following config line:
 
         set noindex