Add/user sbbs_t::getuseron() for better user.tab error logging/handling

Looking into potential causes of issue #843, I found several instances where
we call getuserdat() without checking the return value and restoring the
useron.number to the current user number upon error: getuserdat() zeroes out
the user struct/number upon error, a bad API choice made 23 years ago.

Replace those instances with calls to sbbs_t::getuseron() which logs any
open/lock/read failures of the user.tab and does not modify/zero-out the
sbbs_t::useron struct upon error.