Add in backwards compat to extract _BK_USER from old clients so we get a BK_USER in triggers et al.
Add in backwards compat to extract _BK_USER from old clients so
we get a BK_USER in triggers et al.

bk: 3c912fe7Q8anijrN6owdK231IcUqvg
bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper