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

include/curl/urlapi.h

@@ -64,6 +64,7 @@ typedef enum {
   CURLUE_BAD_USER,            /* 29 */
   CURLUE_LACKS_IDN,           /* 30 */
   CURLUE_TOO_LARGE,           /* 31 */
+  CURLUE_BACKSLASH,           /* 32 */
   CURLUE_LAST
 } CURLUcode;