lib: sanitize conditional exclusion around MIME

lib: sanitize conditional exclusion around MIME

The introduction of CURL_DISABLE_MIME came with some additional bugs:
- Disabled MIME is compiled-in anyway if SMTP and/or IMAP is enabled.
- CURLOPT_MIMEPOST, CURLOPT_MIME_OPTIONS and CURLOPT_HTTPHEADER are
  conditioned on HTTP, although also needed for SMTP and IMAP MIME mail
  uploads.

In addition, the CURLOPT_HTTPHEADER and --header documentation does not
mention their use for MIME mail.

This commit fixes the problems above.

Closes #9610