CURLOPT: bump CURLFTP* enums to long, drop casts

[vszakats]

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.

[bagder]

I think the enums still need to be provided, as they have been offered in the header for a long while there is a significant change/risk that there are users using this publicly provided type in source code now.

[vszakats]

I kept the enums in curl/curl.h.
This is a copy in the docs, synced with the updated header.

There is a similar copy case in CURLOPT_HSTS_CTRL,
out of sync with curl/curl.h, ref #17791.

It'd seem safe to delete such copies from the docs, since the
macros are described in the texts anyway. What do you think?

[dfandrich]

FWIW, you can force an enum to be long in C23 (so with gcc >=15) with the syntax enum:long {…}.