Bug#7150: Distinguishing between left and right Alt keys on NextStep/OSX.

Bug#7150: Distinguishing between left and right Alt keys on NextStep/OSX.

* lisp/cus-start.el (all): ns-right-alternate-modifier is new.

* lisp/term/ns-win.el (ns-right-alternate-modifier): New defvar.
(ns-right-option-modifier): New alias for ns-right-alternate-modifier.
(mac-right-option-modifier): New alias for ns-right-option-modifier.

* src/nsterm.m (Qleft): Declare.
(ns_right_alternate_modifier): New variable
(NSRightAlternateKeyMask): New define.
(EV_MODIFIERS): Parse NSRightAlternateKeyMask if
ns_right_alternate_modifier isn't Qleft.
(keyDown): If ns_right_alternate_modifier isn't Qleft, use it
as emacs modifier for NSRightAlternateKeyMask.
(syms_of_nsterm): DEFVAR_LISP ns-right-alternate-modifier.