Fix frame destruction issues and misuse of x_get_atom_name

Fix frame destruction issues and misuse of x_get_atom_name

* src/frame.c (delete_frame): Prevent deleting the dnd frame
while waiting for finish as well.
* src/xselect.c (Fx_get_atom_name): Clean up code and fix
uninitialized use of need_sync.
* src/xterm.c (x_dnd_send_unsupported_drop, x_dnd_send_drop)
(handle_one_xevent): Clean up usage of x_get_atom_name.
(x_get_atom_name): Accept NULL for need_sync.

* src/xterm.h: Update declarations.