IPV6: address scope != scope id

There was a confusion between these: this commit tries to disambiguate them.
- Scope can be computed from the address itself.
- Scope id is scope dependent: it is currently defined as 1-based local
  interface index for link-local scoped addresses, and as a site index(?) for
  (obsolete) site-local addresses. Linux only supports it for link-local
  addresses.
The URL parser properly parses a scope id as an interface index, but stores it
in a field named "scope": confusion. The field has been renamed into "scope_id".
Curl_if2ip() used the scope id as it was a scope. This caused failures
to bind to an interface.
Scope is now computed from the addresses and Curl_if2ip() matches them.
If redundantly specified in the URL, scope id is check for mismatch with
the interface index.

This commit should fix SF bug #1451.