Added option to link to avahi directly, rather than calling it externally.

Added the option to link to avahi directlly, rather than calling the external "avahi-publish-service" command. This is cleaner, and it means the service broadcast is reliably removed on exit. To enabled, uncomment the line setting LINK_AVAHI in the Makefile. Requires the libavahi-client-dev package on Ubuntu/Debian to compile.