cgroup: refactor for cgroups v2, much cleaner + unified hieararchy

Since the cgroups support in Finit is not yet officially released, we
decided to change the back-end to use cgroups v2 instead of the aging
and rather clumsy cgroups v1.  Even this initial refactor is a lot
easier to read and understand, more can still be done since there's
a lot of concepts, data values and the ilk that can now be shared
between different controllers.

Still ToDo: inotify for cgroup.events to clean up leaf nodes, which
            in cgroups v1 was handled by cgreaper.sh.  This is fixed
	    in the next commit in the series.

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

src/finit.h

@@ -46,7 +46,7 @@
 #endif
 
 #ifndef FINIT_CGPATH
-#define FINIT_CGPATH            "/sys/fs/cgroup/finit"
+#define FINIT_CGPATH            "/sys/fs/cgroup"
 #endif
 
 #define LOGIT_PATH              FINIT_LIBPATH_ "/logit"