De-duplicate directory names passed through to the -d parameter.

Auto-generate directory names if not specified so that they don't clash with pre-existing ones.  This should make it less hassle to quickly generate a new set of replicas without needing to worrying about previous sets.

Update port-collision error message to inform user that omitting the -p parameter will auto-generate ports.  Also add a note about this to the README.

Always output standby directory name when populating it.

Also update README to describe behaviour when passing no parameters.