Stop using GTK3-deprecated gdk_get_display().

Stop using GTK3-deprecated gdk_get_display().

The new way is gdk_display_get_name(gdk_display_get_default()), which
returns a const char * rather than a char *, so I've also had to
fiddle with the prototype and call sites of get_x_display().

(Also included gtkcompat.h into uxputty.c, since that wanted to call
gdk_get_display() but didn't previously include it.)