urlapi: add CURLU_NO_GUESS_SCHEME

urlapi: add CURLU_NO_GUESS_SCHEME

Used for extracting:

- when used asking for a scheme, it will return CURLUE_NO_SCHEME if the
  stored information was a guess

- when used asking for a URL, the URL is returned without a scheme, like
  when previously given to the URL parser when it was asked to guess

- as soon as the scheme is set explicitly, it is no longer internally
  marked as guessed

The idea being:

1. allow a user to figure out if a URL's scheme was set as a result of
  guessing

2. extract the URL without a guessed scheme

3. this makes it work similar to how we already deal with port numbers

Extend test 1560 to verify.

Closes #13616