Fix multi-leg promotions

Fix multi-leg promotions

Two-leg promotions were sent as gibberish to the engine (and thus
rejected). It was also pretty annoying that XBoard's own idea of
what should be promotions was still applied even when highlighting
denied promotions (because the zone was different than assumed, or
a promotion-on-entry rule applies). Now the legality markers from
the GUI move generator are different from the red and yellow markers
from an engine highlight command, and the latter are taken as a denial
of promotion, and will suppress it.