Refactor sys condition plugin into a standalone keventd

This patch is a refactor of the prototype sys condition plugin.  It
moves most of the logic to monitor kernel events into a keventd that,
currently only, sets and clears the sys/pwr/ac condition.  The sys
plugin itself is now only a monitor of conditions and ensures Finit
follows them.  This is a lot more secure and moves (at least one piece
of) netlink processing out from PID 1.

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

src/.gitignore

@@ -1,6 +1,7 @@
 .deps/*
 finit
 initctl
+keventd
 logit
 reboot
 sulogin