This pam plugin never used GDBus because it transparently uses threads
which do not work well with a lot of PAM applications. But even settling
on the "still better to use than plain dbus library" dbus-glib wasn't
without problems, as any use or initialisation of GIO sockets would
modify signal handler for signals such as SIGPIPE (see gio/gsocket.c).

Many years later, sd-bus is a more modern alternative to the bare dbus
library with a better API.

This includes:
- Removing use of gboolean, guint, g_new0() and many glib string helpers
- Simplifying debug logging
- Marking user-facing messages to be translated