Lower the roundtrip overhead of x-get-atom-name by 50% for common atoms

Lower the roundtrip overhead of x-get-atom-name by 50% for common atoms

* src/xselect.c (Fx_get_atom_name):
* src/xterm.c (x_dnd_begin_drag_and_drop): Use that instead.  Don't
sync to handle errors if using `x_get_atom_name' resulted in no
protocol request being made.
(x_get_atom_name): New function.
* src/xterm.h: Update prototypes.