Add several fixes to make wmbubble work on Debian GNU/kFreeBSD

In particular:
 - recognize GNU/kFreeBSD as an option in the Makefile
 - call init_stuff if __FreeBSD_kernel__ is defined instead of just __FreeBSD__
 - use sysctl instead of kvm_read for system_cpu; inspired by FreeBSD patch for
   wmcube-gdk by KOIE Hidetaka (http://lists.freebsd.org/pipermail/
   freebsd-ports-bugs/2008-January/135006.html)