1. when de-activating the passthru socket, give the passthru_thread some
   cycles to copy any remaining socket data into the outbuf before we let
   the node_thread continue on and spew its own data into the outbuf.
   This fixes the problem of final messages of a file transfer protocol
   being intermixed with BBS data (prompts, menus, and such) and causing
   file download finalization issues.
2. Don't read from the passthru socket until there is enough room in the
   outbuf for the maximum possible telnet-expanded read size. This fixes
   streaming download protocol (ZMODEM and YMODEM-G) errors.

Also, for good measure, check the passthru_socket for writability before
send()ing on it and log a warning if it's ever not writable.