afp: Try to find credentials even if user is not specified

Currently, when you try to mount AFP share and don't specify username in the URI, the g_vfs_keyring_lookup_password() call is skipped and thus you have to specify the credentials manually even if "Remember Password Forever" was used last time. This is wrong and other backends in the same situation tries to use the stored credentials. This is especially annoying for locations which are propagated over DNS-SD. Let's change this.

Closes: #352 (closed)