Introduce x_uncatch_errors_after_check to reduce XSync calls.

Introduce x_uncatch_errors_after_check to reduce XSync calls.

Both x_had_errors_p and x_check_errors call XSync, so if they're
immediately followed by x_uncatch_errors, its XSync call will be
redundant, resulting in a wasted round trip to the X server.

* src/xterm.c (x_uncatch_errors_after_check): New routine; a copy of
x_uncatch_errors without the XSync call.
(XTmouse_position, x_wm_supports):
* src/xfns.c (x_set_mouse_color):
* src/xmenu.c (Fx_menu_bar_open_internal):
* src/xselect.c (x_own_selection, x_get_foreign_selection):
(Fx_get_atom_name): Call it instead of x_uncatch_errors.
* src/xterm.h (x_uncatch_errors_after_check): Declare.