For the TCP transport, put the listening socket in non-blocking

For the TCP transport, put the listening socket in non-blocking
mode.  This addresses a well-known race condition that can cause
servers to hang in accept().  The relevant case is when somebody
connects to the server and then immediately kills the connection
by sending a TCP reset.  On the server this causes select to report
a ready condition on the socket, after which the accept call blocks
because there is no longer any pending connection to accept.

In -current there is already a work-around for this in the kernel.
It was merged into -stable some time ago, but then David Greenman
reverted it because it seemed to be causing a socket leak in some
cases.  (See uipc_socket.c revision 1.51.2.3.)  Hence this userland
fix is needed in -stable, and I plan to merge it into that branch
soon because it fixes a potential DoS attack.  It may also be needed
in -current if the suspected socket leak turns out to be real.  In
any case, after thinking it over I believe the fix belongs in
userland.  An application shouldn't assume that a ready return from
select guarantees that the subsequent I/O operation cannot block.
A lot can happen between the select and the accept.

A similar fix should most likely be applied to the Unix domain
socket transport too.

Submitted by:	peter
Reviewed by:	jdp