mutter-launch: remove PAM usage

Left-overs from the weston-launch import

https://bugzilla.gnome.org/show_bug.cgi?id=707584
util: fix warnings when compiling gnome-shell-wayland

Replace our version of the gettext macros with those in glib,
so that they don't conflict with gnome-shell.

https://bugzilla.gnome.org/show_bug.cgi?id=707851
wayland: fix deprecation warnings

Replace the deprecated APIs with the modern variants (which
also give us full control on the versioning).
Also, note that we only support version 2 of wl_seat. Version
3 is for multiple pointer/keyboards for a client, but
we don't implement that yet. If a client requests version 3
of wl_seat or make a version 3 request, it will get a protocol error.

https://bugzilla.gnome.org/show_bug.cgi?id=707851
wayland: fix interface versioning

Add MIN(...) with the interface version actually implemented
to all resource constructor, so that we never risk seeing requests
we don't implement (and consequently segfault)

https://bugzilla.gnome.org/show_bug.cgi?id=707851
wayland: use symbolic constants for interface version

Replace magic numbers scattered around the code with proper
macros collected in one header file.

https://bugzilla.gnome.org/show_bug.cgi?id=707851