CURLOPT: bump CURLFTP* enums to long, drop casts

This patch bumps the size of these constants from `int` to `long`, while
keeping their actual values the same. It may cause incompatibilities in
user code, requiring the bump of holder variables and/or adding casts:

- CURLFTP_CREATE_DIR
- CURLFTP_CREATE_DIR_NONE
- CURLFTP_CREATE_DIR_RETRY
- CURLFTPAUTH_DEFAULT
- CURLFTPAUTH_SSL
- CURLFTPAUTH_TLS
- CURLFTPMETHOD_DEFAULT
- CURLFTPMETHOD_MULTICWD
- CURLFTPMETHOD_NOCWD
- CURLFTPMETHOD_SINGLECWD
- CURLFTPSSL_CCC_ACTIVE
- CURLFTPSSL_CCC_NONE
- CURLFTPSSL_CCC_PASSIVE

Also:
- keep existing casts within the documentation to make sure it applies
  to older curl versions as well.

Closes #17797

src/tool_paramhlp.c

@@ -620,7 +620,7 @@ ParameterError add2list(struct curl_slist **list, const char *ptr)
   return PARAM_OK;
 }
 
-int ftpfilemethod(struct OperationConfig *config, const char *str)
+long ftpfilemethod(struct OperationConfig *config, const char *str)
 {
   if(curl_strequal("singlecwd", str))
     return CURLFTPMETHOD_SINGLECWD;
@@ -635,7 +635,7 @@ int ftpfilemethod(struct OperationConfig *config, const char *str)
   return CURLFTPMETHOD_MULTICWD;
 }
 
-int ftpcccmethod(struct OperationConfig *config, const char *str)
+long ftpcccmethod(struct OperationConfig *config, const char *str)
 {
   if(curl_strequal("passive", str))
     return CURLFTPSSL_CCC_PASSIVE;