Treat all the wayland connection as the same display

Treat all the wayland connection as the same display

On Wayland,
even if without WAYLAND_DISPLAY, --display DISPLAY works, but
gdk_display_get_name() always return "wayland-0", which may be
different from DISPLAY.
If with WAYLAND_DISPLAY, then it always returns WAYLAND_DISPLAY.
So pgtk emacs is confused and enter multi display environment.
To workaround this situation, treat all the wayland-* as the same
display.

* src/pgtkfns.c (is_wayland_display): New function to decide it is a
wayland connection.
(pgtk_display_info_for_name): If both are wayland connection, then
they are the same display.