MAJOR: make use of conn_{data|sock}_{poll|stop|want}* in connection handlers

MAJOR: make use of conn_{data|sock}_{poll|stop|want}* in connection handlers

This is a second attempt at getting rid of FD_WAIT_*. Now the situation is
much better since native I/O handlers can directly manipulate the FD using
fd_{poll|want|stop}_* and the connection handlers manipulate connection-level
flags using the conn_{data|sock}_* equivalent.

Proceeding this way ensures that the connection flags always reflect the
reality even after data<->handshake switches.