It is unnecessary because vfd_open will check the handle and return a valid pseudo-fd

for compatibility with stdlib's socket module

for compatibility with stdlib's socket module

add server.DatagramServer; modify baseserver.BaseServer in such a way that makes it a good base class for both StreamServer and DatagramServer

baseserver.py:
- BaseServer no longer accepts 'backlog' parameter. It is now done by StreamServer.
- BaseServer implements start_accepting() and stop_accepting() methods
- BaseServer now implements "termporarily stop accepting" strategy
- BaseServer now has _do_read method which does everything except for actually calling accept()/recvfrom()
- pre_start() method is renamed to init_socket()
- renamed _stopped_event to _stop_event
- 'started' is now a read-only property (which actually reports state of _stop_event)
- post_stop() method is removed
- close() now sets _stop_event(), thus setting 'started' to False, thus causing serve_forever() to exit
- _tcp_listener() function is moved from baseserver.py to server.py
- added 'fatal_errors' class attribute which is a tuple of all errnos that should kill the server

server.py:
- StreamServer: 'ssl_enabled' is now a read-only property