Fix Windows-only postmaster code to reject a connection request and continue,

Fix Windows-only postmaster code to reject a connection request and continue,
rather than elog(FATAL), when there is no more room in ShmemBackendArray.
This is a security issue since too many connection requests arriving close
together could cause the postmaster to shut down, resulting in denial of
service.  Reported by Yoshiyuki Asaba, fixed by Magnus Hagander.