feature: use vessel name as mdns serviceName

https://tools.ietf.org/html/rfc6763#section-4.1.1

For the http service prefix the vessel name with SK.
For the SK service types just use the vessel name.

src/mdns.js

@@ -89,27 +89,31 @@ module.exports = function mdnsResponder(app) {
     txt: txtRecord
   }
 
-  const host = app.config.getExternalHostname()
+  const instanceName = getInstanceName(app.signalk)
 
+  const host = app.config.getExternalHostname()
   if (host !== require('os').hostname()) {
     options.host = host
   }
 
-  debug(options)
 
   const ads = []
-  // tslint:disable-next-line: forin
-  for (const i in types) {
-    const type = types[i]
+  types.forEach((type, i) =>  {
     debug(
       'Starting mDNS ad: ' +
-        type.type +
-        ' ' +
-        app.config.getExternalHostname() +
-        ':' +
-        type.port
+      type.type +
+      ' ' +
+      app.config.getExternalHostname() +
+      ':' +
+      type.port
     )
-    const ad = new mdns.Advertisement(type.type, type.port, options)
+    let name
+    if (instanceName) {
+      name = toUtfMaxLength(i === 0 ? `SK ${instanceName}` : instanceName)
+    }
+    const optionsForType = {name, ...options}
+    debug(optionsForType)
+    const ad = new mdns.Advertisement(type.type, type.port, optionsForType)
     ad.on('error', err => {
       console.log(type.type.name)
       console.error(err)
@@ -119,11 +123,30 @@ module.exports = function mdnsResponder(app) {
   }
 
   return {
-    stop: function() {
-      ads.forEach(function(ad) {
+    stop: function () {
+      ads.forEach(function (ad) {
         debug('Stopping mDNS advertisement...')
         ad.stop()
       })
     }
   }
 }
+
+const AD_NAME_MAX_UTF_LENGTH = 63 - 3 //allow prefix 'SK ' for http
+
+function getInstanceName(signalk) {
+  const full = signalk.retrieve()
+  return _.get(full, `${_.get(full, 'self')}.name`)
+}
+
+function toUtfMaxLength(s) {
+  let result = s
+  while (utfLength(result) > AD_NAME_MAX_UTF_LENGTH) {
+    result = result.slice(0, result.length - 1)
+  }
+  return result
+}
+
+function utfLength(s) {
+  return ~-encodeURI(s).split(/%..|./).length
+}