This commit adds a new NextAvailablePortForProcess function that takes a
process ID and then assures unique (non-occupied) port numbers are
returned per process.
This uses a temporary file that contains the latest used port and a
secondary temporary lock file to assure only a single goroutine can
request a new port at a time.

The GenerateProcessUniqueListenerAddresses is intened to be used as a
package-level override for the ListenAddressGenerator variable. We don't
use it by default to make sure we don't break any existing assumptions.