Add an rcNG script for modifying avahi

Rather than avahi changes being scattered throughout the
system in various rc scripts consolidate them into a
central place.  Along the way unify all of the avahi
configs in to one config file.  This prevents mDNS
aware systems from getting confused into thinking there
are multiple systems presenting the services that are
advertised by mDNS.

In addition start to advertise all services that FreeNAS
provides via mDNS.

TODO: Add advertisements for rsync and ftp
Always start mDNS services and dependancies.

FreeNAS is moving to a model where all services (including the
webui) are advertised by mDNS.  In this model it doesn't make
sense to make avahi startup conditional.
Delete harmful sample configs when creating the image.

Avahi creates it's services directory when the package is installed.
There's no need to try and create it manually. However it installs
some suggested config files and that really causes problems when
FreeNAS thinks it owns creating the avahi config, so go ahead and
nuke those sample config files when creating the image.
Regenerate avahi config and reload avahi when neccesary.

The reload verb was added to the avahi-daemon rc script.  In addition
an rc script was added that can generate an avahi services config.
Go ahead and run this anytime it a service that is advertised by avahi
is changed.