urlapi: detect scheme better when not guessing

urlapi: detect scheme better when not guessing

When the parser is not allowed to guess scheme, it should consider the
word ending at the first colon to be the scheme, independently of number
of slashes.

The parser now checks that the scheme is known before it counts slashes,
to improve the error messge for URLs with unknown schemes and maybe no
slashes.

When following redirects, no scheme guessing is allowed and therefore
this change effectively prevents redirects to unknown schemes such as
"data".

Fixes #9503