gprs-context: added OFONO_GPRS_AUTH_METHOD_NONE

This method makes explicit the lack of authentication.

When selected, the username and password are ignored, but they are not
changed in the user-defined properties for the context.
This treatment is necessary to allow setting independently auth_method,
username and password.

This method is also selected implicitly when username is set to
an empty string. Also this selection is done without changing the
user-defined auth_method for the context, so that the behavior is
consistent.

include/gprs-context.h

@@ -57,6 +57,7 @@ enum ofono_gprs_context_type {
 enum ofono_gprs_auth_method {
 	OFONO_GPRS_AUTH_METHOD_CHAP = 0,
 	OFONO_GPRS_AUTH_METHOD_PAP,
+	OFONO_GPRS_AUTH_METHOD_NONE,
 };
 
 struct ofono_gprs_primary_context {