Don't fork for xapian-tcpsrv --one-shot

Don't fork for xapian-tcpsrv --one-shot

We only need to fork so we can handle multiple concurrent connections,
which isn't a concern in this case, so forking just adds overhead and
potentially complicates process management for our caller.

This aligns with the behaviour under __WIN32__ where we use threads
instead of forking, and service the connection from the main thread with
--one-shot.