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>

doc/plugins.md

@@ -43,6 +43,9 @@ For your convenience a set of *optional* plugins are available:
 * *hotplug.so*: Setup and start either udev or mdev hotplug daemon, if
   available.  Enabled by default.
 
+* *mdevd.so*: Set up and start mdevd (mdev replacement).  Enabling this
+  also changes how `hotplug.so` plugin operates.  _Optional plugin._
+
 * *rtc.so*: Restore and save system clock from/to RTC on boot/halt.
   Enabled by default.