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

lib/ftp.c

@@ -2749,7 +2749,7 @@ static CURLcode ftp_pp_statemachine(struct Curl_easy *data,
            requested. Try an FTPS connection now */
 
         ftpc->count3 = 0;
-        switch(data->set.ftpsslauth) {
+        switch((long)data->set.ftpsslauth) {
         case CURLFTPAUTH_DEFAULT:
         case CURLFTPAUTH_SSL:
           ftpc->count2 = 1; /* add one to get next */
@@ -2868,7 +2868,7 @@ static CURLcode ftp_pp_statemachine(struct Curl_easy *data,
          * server, but we do not send one. Let's hope other servers do
          * the same... */
         result = Curl_ssl_cfilter_remove(data, FIRSTSOCKET,
-          (data->set.ftp_ccc == CURLFTPSSL_CCC_ACTIVE));
+          (data->set.ftp_ccc == (unsigned char)CURLFTPSSL_CCC_ACTIVE));
 
         if(result)
           failf(data, "Failed to clear the command channel (CCC)");

lib/setopt.c

@@ -900,7 +900,7 @@ static CURLcode setopt_long(struct Curl_easy *data, CURLoption option,
      */
     if((arg < CURLFTPAUTH_DEFAULT) || (arg >= CURLFTPAUTH_LAST))
       return CURLE_BAD_FUNCTION_ARGUMENT;
-    data->set.ftpsslauth = (unsigned char)(curl_ftpauth)arg;
+    data->set.ftpsslauth = (unsigned char)arg;
     break;
   case CURLOPT_ACCEPTTIMEOUT_MS:
     /*