Implement Betza g modifier for non-final legs

Implement Betza g modifier for non-final legs

The g modifier is implemented as a p that turns a rider into the
corresponding leaper when it hops, and vice versa. This is done by
stripping off any existing range indicator, and making the range
infinite by suffixing '0' when the old range was 1 (whether implied
or explicitly written). Except that for sliders/steppers this is done
by atom-name conversion after stripping off the range. Limited range
thus in general turns into leaper (D -> D0, Dn -> D), but with FWK
W3 -> R and R3 -> W, etc.