Validate and simplify LoginParam struct

This makes sure that under normal circumstances the LoginParam struct
is always fully validated, ensure future use does not have to be
careful with this.

The brittle handling of `server_flags` is also abstraced away from
users of it and is now handled entirely internally, as the flags is
really only a boolean a lot of the flag parsing complexity is removed.
The OAuth2 flag is moved into the ServerLoginParam struct as it really
belongs in there.

src/context.rs

@@ -342,7 +342,7 @@ impl Context {
 
     pub async fn get_info(&self) -> Result<BTreeMap<&'static str, String>> {
         let unset = "0";
-        let l = LoginParam::load_candidate_params(self).await?;
+        let l = LoginParam::load_candidate_params_unchecked(self).await?;
         let l2 = LoginParam::load_configured_params(self).await?;
         let secondary_addrs = self.get_secondary_self_addrs().await?.join(", ");
         let displayname = self.get_config(Config::Displayname).await?;