Add update-avahi

[jwm]

update-avahi is a tool for managing xml service descriptions by storing them in an available-service directory, and symlinking them into /etc/avahi/service, much like the tools apache uses (under Debian, at least), to manage configuration blocks and modules.

It provides a quicker path to getting ssh service advertisment working out of the box, but it's of greatest benefit to maintainers of other package, who will have a target directory to add their own service descriptions, and can use update-avahi in init scripts to advertise a service when the daemon is running.

A couple of implementation notes:

• The script is actually something I wrote for another purpose and generalised. It can also manage the active scripts with an order prefix, but that feature is disabled for avahi.
• I reduced the license boilerplate to a reference to the main license file, as this results in service files that mostly fit into 24 rows of text. Makes reading and editing much easier.