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/Makefile.am

@@ -21,6 +21,10 @@ if BUILD_HOTPLUG_PLUGIN
 libplug_la_SOURCES += hotplug.c
 endif
 
+if BUILD_MDEVD_PLUGIN
+libplug_la_SOURCES += mdevd.c
+endif
+
 if BUILD_MODULES_LOAD_PLUGIN
 libplug_la_SOURCES += modules-load.c
 endif
@@ -72,6 +76,10 @@ if BUILD_HOTPLUG_PLUGIN
 pkglib_LTLIBRARIES += hotplug.la
 endif
 
+if BUILD_MDEVD_PLUGIN
+pkglib_LTLIBRARIES += mdevd.la
+endif
+
 if BUILD_MODULES_LOAD_PLUGIN
 pkglib_LTLIBRARIES += modules-load.la
 endif