Avahi support for Postgresql

Postmaster already has code to announce its services via DNS-SD
(ZeroConf) by using Apple's Bonjour API. This series of patches
implements that capability on top of the Avahi library[1] which
is free software, available for a wider variety of platforms.

I've separated the change set into smaller pieces for easy review. Also
I don't know, if the 4th patch adding subtypes describing Postgresql's
capabilities is wanted by the Postgresql community. Being a desktop guy,
using Postgresql as embedded database, I'd really like to see all of the
patches merged, but I could imagine some opposition against the verbose
nature of the subtypes patch.

Each of the patches as a short description of its purpose.

I couldn't find any ChangeLog files in the repository, and also I didn't
invest much time in investigating how the HISTORY file of distribution
tarballs is assembled. So if the repository contains file I should have
modified for documenting my changes, I apologize for not updating them.

[1] http://www.avahi.org/
--
Mathias Hasselmann <mathias(at)openismus(dot)com>
www.openismus.com