Allow specifying --port-range-begin and --port-range-end

Options ``--port-range-begin`` and ``--port-range-end`` would allow customization of the ports range used by ``portallocator``, and they keep current defaults as specified by constants. Closes #4736.

Some notes on this pull request:
- changes wording of "port range begin" to "port range start"
- uses ``fmt.Sprintf`` to inline real default values in the cli help output