Adding Rendezvous support to postmaster

The attached diffs add Rendezvous support to postmaster when it's
running on Darwin (Mac OS X).

Rendezvous allows services (such as a database server) to advertise
their availability to other hosts on their link-local network, and
client programs to browse the services on the network to see what's
available. It is Apple's implementation of ZEROCONF.

    http://developer.apple.com/macosx/rendezvous/

This allows client programs running on computers that are on the same
link-local network as the postgresql server to automatically find the
server's IP address and port number. This adds great ease-of-use for
end users.

I propose adding a configuration variable to postgresql.conf called
"service_name" that allows the user to specify the name used to
advertise the database service. If this variable isn't defined in the
postgresql.conf file, then no service is advertised. If it is '', then
the computer's "Rendezvous Name" is used (this is similar to the
hostname, see the link below for more info). Otherwise, if it's a
non-empty string, that string is used as the name of the service.

    http://developer.apple.com/qa/qa2001/qa1228.html

If there's already a service on the network with that name, then the
service registration silently fails and no service is registered.

Rendezvous also has the notion of a service type string. It's a bit
like a domain name: I suggest we use "_pgsql._tcp." (another example
would be "_ftp._tcp.").

There's very little code involved: just 1 function call in
PostmasterMain() after postmaster has started listening on a TCP
socket. And 1 addition to the GUC list of configuration variables.

All the code is #ifdef'ed inside HAVE_RENDEZVOUS, which is only defined
in src/include/port/darwin.h. Even once we know the OS is darwin, we
still need to check to make sure that Rendezvous is available, either
by OS version, or by checking for the existence of a needed header
file, etc.

I've included patches for CVS top-of-tree and postgresql-7.3.2.

Thanks!

- Chris