Fixes 2 bugs:
1) an impossible() could occur if you applied a lance against a long worm
because the code uses thitmonst to do the hitting, but didn't set bhitpos,
which is required before calling thitmonst.
Add the missing assignment.
2) applying a lance would never mark a knight as a caitiff. Added a new
check_caitiff function and called it from the 2 existing checks and in
the lance code.
35914ed186