Add "AuthOnlyViaModule" global/user setting

Setting AuthOnlyViaModule on a user causes CheckPass to never return true,
causing all authentication attempts using the configured password to fail, both
on IRC connections and for webadmin. This is useful in situations where an
external module (cyrusauth, certauth, imapauth) handles authentication. Setting
the global AuthOnlyViaModule option causes similar behavior across every
user. If AuthOnlyViaModule is set to true globally, it cannot be overridden
per-user.

Close #1474
Close #331

test/UserTest.cpp

@@ -119,3 +119,33 @@ TEST_F(UserTest, IsHostAllowed) {
             << "Allow-host is " << h.sMask;
     }
 }
+
+TEST_F(UserTest, TestAuthOnlyViaModule) {
+    CUser user("user");
+    user.SetPass("password", CUser::HASH_NONE);
+
+    bool bAuthOnlyViaModuleDefault = CZNC::Get().GetAuthOnlyViaModule();
+
+    CZNC::Get().SetAuthOnlyViaModule(false);
+    user.SetAuthOnlyViaModule(false);
+
+    EXPECT_TRUE(user.CheckPass("password"));
+
+    // user-level only
+    user.SetAuthOnlyViaModule(true);
+    EXPECT_FALSE(user.CheckPass("password"));
+
+    // re-enabling built-in authentication
+    user.SetAuthOnlyViaModule(false);
+    EXPECT_TRUE(user.CheckPass("password"));
+
+    // on at global level, off at user level
+    CZNC::Get().SetAuthOnlyViaModule(true);
+    EXPECT_FALSE(user.CheckPass("password"));
+
+    // on at both levels
+    user.SetAuthOnlyViaModule(true);
+    EXPECT_FALSE(user.CheckPass("password"));
+
+    CZNC::Get().SetAuthOnlyViaModule(bAuthOnlyViaModuleDefault);
+}