CURLOPT: bump `CURLHEADER_*` macros to `long`, drop casts

CURLOPT: bump `CURLHEADER_*` macros to `long`, drop casts

This patch bumps the size of these macros 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:

- CURLHEADER_SEPARATE
- CURLHEADER_UNIFIED

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

Closes #18055