automount: port from CK to systemd-logind

automount: port from CK to systemd-logind

ConsoleKit is being obsoleted by systemd. Accordingly port the CK logic
in the gnome-shell automount manager to systemd-logind APIs.

This makes use of systemd-logind's native C APIs which are much easier
to use than the D-Bus APIs in this case, and much faster too (since they
are synchronous and directly query the kernel for the information we
need). The dependency is compile time optional, and in order to be nice
to the Debian folks g-s compiled with this enabled fill automatically
fall back to CK support on systems lacking systemd.