Embedded web servers behave inconsistently when server.port is set to -1

[wilkinsona]

The behaviour is as follows when starting with server.port=-1:

• Jetty: app starts and stays running, but is not listening for HTTP connections
• Netty: app fails to start as -1 is an illegal port number
• Tomcat: app starts and stays running, but is not listening for HTTP connections
• Undertow: app starts and then stops

Further to the above, the way that autoStart is implemented at the moment is a little odd. When a web server is created with autoStart set to false, calling start() on that server will then have no effect and there's no way to set autoStart back to true so that the web server can be started. In other words, it's not really disabling automatic start, but disabling start altogether.

[wilkinsona]

We'd like to align Netty and Undertow with Jetty and Tomcat. We'll decide if the fix should go in 2.1.x or if it should wait till 2.2 depending on how risky it seems.