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

docs/libcurl/opts/CURLOPT_FTP_CREATE_MISSING_DIRS.md

@@ -21,11 +21,9 @@ CURLOPT_FTP_CREATE_MISSING_DIRS - create missing directories for FTP and SFTP
 ~~~c
 #include <curl/curl.h>
 
-typedef enum {
-  CURLFTP_CREATE_DIR_NONE,
-  CURLFTP_CREATE_DIR,
-  CURLFTP_CREATE_DIR_RETRY
-} curl_ftpcreatedir;
+#define CURLFTP_CREATE_DIR_NONE  0L
+#define CURLFTP_CREATE_DIR       1L
+#define CURLFTP_CREATE_DIR_RETRY 2L
 
 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_FTP_CREATE_MISSING_DIRS,
                           long create);