session_policy_local: Use callback helpers

Use the generic callback helpers to encapsulate struct create_data.
In a later patch we will add some more data elemets to struct
create_data. That is the reason why it is not remove here.
session: Change to "func_t" postfix to callback typedef

Streamline the callback typedefs with the rest of the code.
session: Do not fail when creating default policy configuration

Do not handle the small alloc failures because glib will abort
when memory is tight anyway.
dbus: Refactor callback helper struct

The helper struct will be used by a new dbus function which uses
a different callback signature. Therefore, we need to use a
void pointer for storing the callback.
dbus: Add connman_dbus_get_connnection_unix_user()

The UID is fetched via org.freedesktop.DBus.GetConnectionUnixUser D-Bus
method call.