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

docs/libcurl/curl_url_get.md

@@ -125,6 +125,22 @@ nothing following the hash sign.
 
 (Added in curl 8.8.0)
 
+## CURLU_NO_GUESS_SCHEME
+
+When this flag is used in curl_url_get(), it treats the scheme as non-existing
+if it was set as a result of a previous guess; when CURLU_GUESS_SCHEME was
+used parsing a URL.
+
+Using this flag when getting CURLUPART_SCHEME if the scheme was set as the
+result of a guess makes curl_url_get() return CURLUE_NO_SCHEME.
+
+Using this flag when getting CURLUPART_URL if the scheme was set as the result
+of a guess makes curl_url_get() return the full URL without the scheme
+component. Such a URL can then only be parsed with curl_url_set() if
+CURLU_GUESS_SCHEME is used.
+
+(Added in curl 8.9.0)
+
 # PARTS
 
 ## CURLUPART_URL

docs/libcurl/symbols-in-versions

@@ -1068,6 +1068,7 @@ CURLU_GET_EMPTY                 8.8.0
 CURLU_GUESS_SCHEME              7.62.0
 CURLU_NO_AUTHORITY              7.67.0
 CURLU_NO_DEFAULT_PORT           7.62.0
+CURLU_NO_GUESS_SCHEME           8.9.0
 CURLU_NON_SUPPORT_SCHEME        7.62.0
 CURLU_PATH_AS_IS                7.62.0
 CURLU_PUNY2IDN                  8.3.0