Revert "Use sysctl module instead of libc hacks"

This reverts commit d640eb509fc3da594cec2c52104213a2e5c76c5c.

Conflicts:
	gui/middleware/notifier.py
Use lxml instead of libxml2

We were having memory leaks with libxml2, plus lxml seems to be more
actively maintained and is BSD licensed.
Catch up with the old sysctl API