libssh2_userauth_authenticated: make it work as documented

The man page clearly says it returns 1 for "already authenticated" but
the code said non-zero. I changed the code to use 1 now, as that is also
non-zero but it gets the benefit that it now matches the documentation.

Using 1 instead of non-zero is better for two reasons:

1. We have the opportunity to introduce other return codes in the future for
   things like error and what not.
2. We don't expose the internal bitmask variable value.

src/userauth.c

@@ -179,12 +179,12 @@ libssh2_userauth_list(LIBSSH2_SESSION * session, const char *user,
  * libssh2_userauth_authenticated
  *
  * Returns: 0 if not yet authenticated
- * non-zero is already authenticated
+ *          1 if already authenticated
  */
 LIBSSH2_API int
 libssh2_userauth_authenticated(LIBSSH2_SESSION * session)
 {
-    return session->state & LIBSSH2_STATE_AUTHENTICATED;
+    return (session->state & LIBSSH2_STATE_AUTHENTICATED)?1:0;
 }