HTTP Auth: Add CURLAUTH_ONLY

This is a meta symbol. OR this value together with a single specific
auth value to force libcurl to probe for un-restricted auth and if not,
only that single auth algorithm is acceptable.

For example you can use CURLAUTH_DIGEST|CURLAUTH_ONLY to make libcurl
first probe for what method to use, but yet only consider Digest to be
acceptable.

Using _only_ CURLAUTH_DIGEST without the CURLAUTH_ONLY field, will make
libcurl explicitly use Digest right away and not do any probing.

docs/libcurl/curl_easy_setopt.3

@@ -865,6 +865,10 @@ secure.
 This is a convenience macro that sets all bits except Basic and thus makes
 libcurl pick any it finds suitable. libcurl will automatically select the one
 it finds most secure.
+.IP CURLAUTH_ONLY
+This is a meta symbol. Or this value together with a single specific auth
+value to force libcurl to probe for un-restricted auth and if not, only that
+single auth algorithm is acceptable. (Added in 7.21.3)
 .RE
 .IP CURLOPT_PROXYAUTH
 Pass a long as parameter, which is set to a bitmask, to tell libcurl which

docs/libcurl/symbols-in-versions

@@ -20,6 +20,7 @@ CURLAUTH_DIGEST_IE              7.19.3
 CURLAUTH_GSSNEGOTIATE           7.10.6
 CURLAUTH_NONE                   7.10.6
 CURLAUTH_NTLM                   7.10.6
+CURLAUTH_ONLY                   7.21.3
 CURLCLOSEPOLICY_CALLBACK        7.7
 CURLCLOSEPOLICY_LEAST_RECENTLY_USED 7.7
 CURLCLOSEPOLICY_LEAST_TRAFFIC   7.7

include/curl/curl.h

@@ -585,6 +585,9 @@ typedef enum {
 #define CURLAUTH_GSSNEGOTIATE (1<<2)  /* GSS-Negotiate */
 #define CURLAUTH_NTLM         (1<<3)  /* NTLM */
 #define CURLAUTH_DIGEST_IE    (1<<4)  /* Digest with IE flavour */
+#define CURLAUTH_ONLY         (1<<31) /* used together with a single other
+                                         type to force no auth or just that
+                                         single type */
 #define CURLAUTH_ANY (~CURLAUTH_DIGEST_IE)  /* all fine types set */
 #define CURLAUTH_ANYSAFE (~(CURLAUTH_BASIC|CURLAUTH_DIGEST_IE))