By default, "*" means "All IPv4 addresses" with
tor_addr_parse_mask_ports, so I won't break anything.  But if the new
EXTENDED_STAR flag is provided, then * means "any address", *4 means
"any IPv4 address" (that is, 0.0.0.0/0), and "*6" means "any IPv6
address" (that is, [::]/0).

This is going to let us have a syntax for specifying exit policies in
torrc that won't drive people mad.

Also, add a bunch of unit tests for tor_addr_parse_mask_ports to test
these new features, and to increase coverage.