(apparent cause of Win32 lock-up when closing socket from non-owner thread)
using bool terminate_server instead, which doesn't abort select(), so using
shorter default sem_chk_freq value (2 seconds vs 5).
resolve_ip() now supports user@[ip.address] notation.
bounce() now always logs reason for bounce.
pop3 server no longer logs uninitialized password  for failed APOP logins.