Exit early on Wayland sessions for now

To celebrate my 100th xfce4-screensaver startup crash on Xfce Wayland session 😂, I am looking at cinnamon-screensaver's wrapper script and wonder if it is okay to do something similar here for now, given !28 won't likely happen in 4.20.

I imagine it will be adding something like this right after after every (?) int main and before any GSMonitor declarations:

const char *session_type = g_getenv ("XDG_SESSION_TYPE");
if (g_strcmp0 (session_type, "wayland") == 0) {
    g_critical ("xfce4-screensaver is disabled in wayland sessions. Exiting.");
    exit(1);
}

(Though this will definitely needs to be reverted in !28)