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

include/curl/curl.h

@@ -1024,8 +1024,8 @@ typedef enum {
 } curl_ftpmethod;
 
 /* bitmask defines for CURLOPT_HEADEROPT */
-#define CURLHEADER_UNIFIED  0
-#define CURLHEADER_SEPARATE (1<<0)
+#define CURLHEADER_UNIFIED  0L
+#define CURLHEADER_SEPARATE (1L<<0)
 
 /* CURLALTSVC_* are bits for the CURLOPT_ALTSVC_CTRL option */
 #define CURLALTSVC_READONLYFILE (1L<<2)