Fix #195: add mdevd plugin as complement to hotplug.sh

This patch adds the mdevd[1] plugin from Andy's fork of Finit, only
minor whitespace adjustment and group fix introduced, along with
updates to configure.ac and the documentation.

Currently the hotplug.sh calls mdevd-coldplug (instad of mdev -s), but
since mdevd is not guaranteed to have started and be ready, it is also
started with mdevd -C.  A better solution is to use the s6 notify code
to synchronize the start of mdevd-coldplug.  Support for this is coming
soon to Finit.

[1]: https://github.com/hongkongkiwi/finit/blob/master/plugins/mdevd.c

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

plugins/hotplug.c

@@ -39,6 +39,19 @@ static void setup(void *arg)
 		return;
 	}
 
+	/*
+	 * If the mdevd plugin is enabled and loaded we assume this
+	 * system is mdevd based, skipping below udev/mdev checks.
+	 */
+	if (plugin_find("mdevd")) {
+		path = which("mdevd-coldplug");
+		if (path) {
+			run_interactive(path, "Populating device tree");
+			free(path);
+		}
+		return;
+	}
+
 	/*
 	 * Populate /dev and prepare for runtime events from kernel.
 	 * Prefer udev if mdev is also available on the system.
@@ -90,12 +103,13 @@ static void setup(void *arg)
 	}
 }
 
+/* mdevd optional, if installed & loaded changes behavior of setup() */
 static plugin_t plugin = {
 	.name = __FILE__,
 	.hook[HOOK_BASEFS_UP] = {
 		.cb  = setup
 	},
-	.depends = { "bootmisc", "modprobe" },
+	.depends = { "bootmisc", "mdevd", "modprobe" },
 };
 
 PLUGIN_INIT(plugin_init)