krb5: fix detecting channel binding feature

Use the already detected `gssapi/gssapi_krb5.h` MIT Kerberos header
to pull in `gssapi_ext.h`, which in turn sets `GSS_C_CHANNEL_BOUND_FLAG`
if supported. Channel binding is present in MIT Kerberos 1.19+.

Also:
- lib: de-duplicate GSS-API header includes.
- vauth: de-duplicate `urldata.h` includes.
- drop interim feature macro in favor of the native GSS one.

Assisted-by: Max Faxälv
Reported-by: Max Faxälv
Bug: https://github.com/curl/curl/pull/19164#issuecomment-3551687025
Follow-up to 8616e5aada #19164
Closes #19603
Closes #19760

lib/vauth/digest.c

@@ -33,7 +33,6 @@
 
 #include "vauth.h"
 #include "digest.h"
-#include "../urldata.h"
 #include "../curlx/base64.h"
 #include "../curl_hmac.h"
 #include "../curl_md5.h"