Linux: Use /sys/class/tty/tty0/active for VT change notifications

This prevents Linux from spawning 63 threads. Now there's one
monitor thread that watches for the active VT to change.
Based on the VT_WAITEVENT patch by Kan-Ru Chen.
Fixes:
https://github.com/ConsoleKit2/ConsoleKit2/issues/1
https://bugs.freedesktop.org/show_bug.cgi?id=17720
https://bugs.launchpad.net/ubuntu/+source/consolekit/+bug/148454