add snake configuration options, only link in HAL libraries in hgTracks,
temporarily(?) remove support for snakes based on chains #10637
Merge branch 'master' of hgwdev.cse.ucsc.edu:/data/git/kent
oops... forgot to add snakeUi.h
double oops... need snakeUi.c too, obvs
Merge remote-tracking branch: upstream/master