Adds the mac_ctrl_key=ctrl/cmd/both preference

... so that mac users can have their ctrl keys back. The default is set to 'both' to match existing behavior, but 'cmd' will work like other editors on the mac. For example, Ctrl-A will move the cursor to the beginning of the line while Cmd-A will select the entire buffer.

I'm open to suggestions for an alternate setting name and set of values.