* xterm.c: Use EMACS_INT for Emacs modifiers, and int for X modifiers.

* xterm.c: Use EMACS_INT for Emacs modifiers, and int for X modifiers.

Before, the code was not consistent.  These values cannot exceed
2**31 - 1 so there's no need to make them unsigned.
(x_x_to_emacs_modifiers): Accept int and return EMACS_INT.
(x_emacs_to_x_modifiers): Accept EMACS_INT and return int.
(x_x_to_emacs_modifiers, x_emacs_to_x_modifiers): Reject non-integers
as modifiers.
* xterm.h (x_x_to_emacs_modifiers): Adjust to signature change.