Improve postmaster's behavior if an accept() call fails. Because the server

Improve postmaster's behavior if an accept() call fails.  Because the server
socket is still read-ready, the code was a tight loop, wasting lots of CPU.
We can't do anything to clear the failure, other than wait, but we should give
other processes more chance to finish and release FDs; so insert a small sleep.
Also, avoid bogus "close(-1)" in this case.  Per report from Jim Nasby.