return >0 from fr_value_box_from_substr()

return >0 from fr_value_box_from_substr()

mainly for ip addresses and prefixes.  And also parse scopes for
DHCPv6.  Line 20 of src/tests/unit/protocols/dhcpv6/addresses.txt
says that "scopes are ignored".

However, the only reason that the previous tests succeeded was that
fr_value_box_from_substr() was returning 0, which meant that
fr_value_box_from_str() didn't check that the entire string was
being parsed.

A first-pass fix was in commit 8a1f819e7f7, but that used an extra
sbuff, instead of just re-using our_in, among other faults.