urlapi: improved return codes

- add CURLUE_BACKSLASH that can be returned when a backslash was used
  where a forward one probably was intended.

- make CURLUE_NO_HOST higher priority than port number errors for URLs
  without hostname. Like in "http://::1"

- shortened some URL parser error strings

Extend test 1560 to verify.

Reported-by: kit-ty-kate on github
Fixes #22337
Closes #22408

docs/libcurl/libcurl-errors.md

@@ -724,6 +724,11 @@ libcurl lacks IDN support.
 
 A value or data field is larger than allowed.
 
+## CURLUE_BACKSLASH (32)
+
+Found a backslash character where a forward slash was expected. URL separators
+are forward slashes (`/`), not backslashes (`\`).
+
 # CURLHcode
 
 The header interface returns a *CURLHcode* to indicate when an error has

docs/libcurl/symbols-in-versions

@@ -1113,6 +1113,7 @@ CURLU_PUNY2IDN                  8.3.0
 CURLU_PUNYCODE                  7.88.0
 CURLU_URLDECODE                 7.62.0
 CURLU_URLENCODE                 7.62.0
+CURLUE_BACKSLASH                8.22.0
 CURLUE_BAD_FILE_URL             7.81.0
 CURLUE_BAD_FRAGMENT             7.81.0
 CURLUE_BAD_HANDLE               7.62.0