Refactor login packet handler for more flexible auth (#298)

This PR enables auth cookies to be used simultaneously with plaintext paasswords sent in the cookie authID field.

* Hoist a bunch of checks from the login packet handler into helper functions.
* Rename the LoginType enum to AuthMethod and distinguish it from the iLoginType packet field (see comment in code for why these should be decoupled).
* If the provided token does not pass the cookie check and password auth is enabled, treat it as a plaintext password and authenticate if it is correct.

src/main.cpp

@@ -222,6 +222,17 @@ time_t getTimestamp() {
     return (time_t)value.count();
 }
 
+// timing safe strcmp implementation for e.g. cookie validation
+int timingSafeStrcmp(const char* a, const char* b) {
+    int diff = 0;
+    while (*a && *b) {
+        diff |= *a++ ^ *b++;
+    }
+    diff |= *a;
+    diff |= *b;
+    return diff;
+}
+
 // convert integer timestamp (in s) to FF systime struct
 sSYSTEMTIME timeStampToStruct(uint64_t time) {