Add mDNS discovery for SimpleHome devices

[cweiske]

With that in place, devices can use avahi-daemon to get their SimpleHome API services automatically discovered by HomeApp.

Example avahi daemon service file:

<?xml version="1.0"?>
<service-group>
  <name>Lamps for HomeApp</name>
  <service protocol="ipv4">
    <type>_simplehome._tcp</type>
    <port>80</port>
    <txt-record>url=http://simpleapi-homeapp.example.org/</txt-record>
  </service>
</service-group>

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The TXT record with "url" property is optional. If not specified, the IP address of the server will be used by HomeApp.

Specifying the url lets you announce multiple services on the same host.