• Call db_setup_paging() for traceall.
• Make it so one can't call db_setup_paging() if it has already been called

before. traceall needs this, or else the db_setup_paging() call from
db_trace_thread() will reset the printed line number, and override its
argument.
This is not perfect for traceall, because even if one presses 'q' while in
the middle of printing a backtrace it will finish printing the backtrace
before exiting, as db_trace_thread() won't be notified it should stop, but
it is hard to do better without reworking the pager interface a lot more.