Add separate key mappings for left/right control/command on Nextstep (Bug#7458).

Add separate key mappings for left/right control/command on Nextstep (Bug#7458).

* lisp/cus-start.el (all): Add ns-right-control-modifier and
ns-right-command-modifier.

* lisp/term/ns-win.el (ns-right-control-modifier)
(ns-right-command-modifier): Defvar them.

* src/nsterm.m (ns_right_command_modifier, ns_right_control_modifier):
Define (Bug#7458).
(NSRightCommandKeyMask, NSRightControlKeyMask): Define (Bug#7458).
(EV_MODIFIERS): Check for NSRightCommandKeyMask and
NSRightControlKeyMask also (Bug#7458).
(keyDown): Ditto (Bug#7458).
(syms_of_nsterm): Defvar ns-right-command-modifier and
ns-right-control-modifier (Bug#7458).