use socket I/O (e.g. sexyz, door32.sys doors, sync-xsdk doors) - not just when
the client is connected via SSH. This solves a number of problems:
- programs that change socket options
- programs that don't support non-CP437 charsets (e.g. PETSCII, UTF-8)
- programs that don't support Telnet-IAC escaping

This greatly simplifies (reduces the need for) input_thread locking.

The passthru_thread now handles chunks of up to 4000 bytes per transfer
between socket and ringbuf, instead of a char at a time, greatly improving the
performance.