For some reason, the old login class code didn't seem to be working here.

For some reason, the old login class code didn't seem to be working here.
I suspect it was because the child exec code's parent was doing the
initial lookups, then forking, then doing other things (possibly trashing
the static data in the getpw*() buffer), then attempting to dereference
*pwd and *lc.  Also, no error checking appeared to be done - I've allowed
it to fall back to the old "become user" code on critical failure rather than
risk running a user's cron jobs as root.