[http] Gracefully handle offers of multiple authentication schemes

Servers may provide multiple WWW-Authenticate headers, each offering a
different authentication scheme.  We currently fail the request as
soon as we encounter an unrecognised scheme, which prevents subsequent
offers from succeeding.

Fix by silently ignoring headers for schemes that we do not recognise.
If no schemes are recognised then the request will eventually fail
anyway due to the 401 response code.

If multiple schemes are supported, arbitrarily choose the scheme
appearing first within the response headers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/httpauth.c

 static int http_parse_www_authenticate ( struct http_transaction *http,
 					 char *line ) {
 	struct http_www_authenticate_field *field;
+	struct http_authentication *auth;
 	char *name;
 	char *key;
 	char *value;
 	}
 
 	/* Identify scheme */
-	http->response.auth.auth = http_authentication ( name );
-	if ( ! http->response.auth.auth ) {
+	auth = http_authentication ( name );
+	if ( ! auth ) {
 		DBGC ( http, "HTTP %p unrecognised authentication scheme "
 		       "\"%s\"\n", http, name );
-		return -ENOTSUP;
+		/* Ignore; the server may offer other schemes */
+		return 0;
 	}
 
+	/* Use first supported scheme */
+	if ( http->response.auth.auth )
+		return 0;
+	http->response.auth.auth = auth;
+
 	/* Process fields */
 	while ( ( key = http_token ( &line, &value ) ) ) {
 		for ( i = 0 ; i < ( sizeof ( http_www_auth_fields ) /