TCP option assign hook doesn't work well if option not supported

macOS does not support the socket option TCP_USER_TIMEOUT.  Yet, I can 
start a server with postgres -D ... --tcp-user-timeout=100 without a 
diagnostic.  Only when I connect I get a log entry

LOG:  setsockopt(TCP_USER_TIMEOUT) not supported

Perhaps the logic in pq_settcpusertimeout() should be changed like this:

  int
  pq_settcpusertimeout(int timeout, Port *port)
  {
+#ifdef TCP_USER_TIMEOUT
     if (port == NULL || IS_AF_UNIX(port->laddr.addr.ss_family))
         return STATUS_OK;

-#ifdef TCP_USER_TIMEOUT
     if (timeout == port->tcp_user_timeout)
         return STATUS_OK;

So that the #else branch that is supposed to check this will also be run 
in the postmaster (where port == NULL).

Or perhaps there should be a separate GUC check hook that just does

#ifndef TCP_USER_TIMEOUT
     if (val != 0)
         return false;
#endif
     return true;

The same considerations apply to the various TCP keepalive settings, but 
since those are widely supported the unsupported code paths probably 
haven't gotten much attention.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services