Fix segfaults in GLib applications when the PAM module is unloaded or reloaded.

We need to use copies of strings instead of static string for creating GQuarks that are used for error handling. If this library is unloaded but GLib isn't, static strings won't be available afterwards so they can cause a segfault.

https://developer.gnome.org/glib/stable/glib-Quarks.html#g-quark-from-static-string