Mac: modifier key remapping revised [patch]

[David Reitter]

Hi,

the current state of affairs makes it difficult to map the standard Mac 
modifier keys (Ctrl, Option, Command) to Emacs modifiers.

At this point, the default is that the command key is Meta, but by 
setting mac-command-key-is-meta to nil, one can map Option to Meta.
An additional variable, mac-reverse-ctrl-meta, allows to swap Ctrl and 
(whatever is) Meta.
Finally, a patch introduced in November last year allows us to map 
Option to whatever (e.g. Hyper or Meta) via the variable 
mac-option-modifier.

This is complicated, and in effect does not allow me to map, for 
example Command to Hyper, which makes a lot of sense if you want to 
assign something to Command-X, without overriding M-x (or A-x) and 
without consequences for the choice of Option.

Therefore I suggest to a) deprecate mac-command-key-is-meta and 
mac-reverse-ctrl-meta, and b) introduce mac-control-modifier and 
mac-command-modifier which allow, analogous to the recent 
mac-option-modifier, free mapping of the modifier keys.

All of a sudden, the modifier key business becomes simple and 
straightforward:

(setq mac-option-modifier 'meta)
(setq mac-control-modifier 'ctrl)
(setq mac-command-modifier 'hyper)

In order to preserve backwards-compatibility, the two deprecated 
variables remain in effect unless at least one of the above 
mac-*-modifier variables is set to non-nil.
Problems should only occur if somebody uses the old mac-option-modifier 
in combination with the deprecated variables; but the option variable 
has been introduced only recently and hasn't seen significant uptake 
yet (google it!).

The attached patch implements this behavior, it applies against today's 
catch (of macterm.c).

mac-modifier-keys.new.patch
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