The gdm-session-worker is a process used for managing interaction
with PAM. PAM modules can do weird things to the process they run
in, so GDM segregrates all PAM conversations in their own independent
"worker" subprocesses.

This commit moves gdm-session-worker away from using dbus-glib
to using gdbus instead.

https://bugzilla.gnome.org/show_bug.cgi?id=622888