feat: add support of pam authentication

Add naive implementation of user password check by calling su command.
Also change some authentication method to require username to be string
instead of optional string

docs/ahriman.core.rst

@@ -52,6 +52,14 @@ ahriman.core.tree module
    :no-undoc-members:
    :show-inheritance:
 
+ahriman.core.util module
+------------------------
+
+.. automodule:: ahriman.core.util
+   :members:
+   :no-undoc-members:
+   :show-inheritance:
+
 ahriman.core.utils module
 -------------------------