Warn when ports are not available

Recently ran into a number of issues at a customer because the system they were trying to install Couchbase on had many of our ports already in use.

Two ways to improve upon this:

-In the current installer we list out all the ports required, but it would be much more helpful if we could actually call out any ports that were already in use.  In the case of Windows (which is where this problem manifested) we can ask whether they want to continue with the installation knowing that there will be problems starting up, and in the case of Linux simply call it out as part of the RPM install.

-In the UI it would be exceedingly helpful to have a warning in the setup wizard that warned of any port conflicts.  (I'm avoiding asking for a continuous check via ns_server)