Better support for unknown URL schemes

Better support for unknown URL schemes

Squid already contains AnyP::PROTO_UNKNOWN support for unknown protocols
but currently does not preserve the actual string value received for them.

This adds a textual representation ('image') to the UriScheme object to
fill that gap and ensure that all URL representations (ie cache keys,
logs and outgoing messages) are generated with the scheme string as it
was received rather than implicitly via a registered protocol type.

Future work:
* add ACL support for arbitrary scheme names
* support for comparisons of unknown schemes