guri: Always prepend `//` to the host when building a URI

This is needed to distinguish the host (‘authority’ in the terms of RFC 3986) from a path if a scheme is not present.

It can be seen from the grammar in RFC 3986 (https://tools.ietf.org/html/rfc3986#appendix-A) that authority only ever appears after "//".

Spotted by Simon McVittie in !1606 (comment 884893).

Signed-off-by: Philip Withnall withnall@endlessm.com