systemd: Add function stubs.

Signed-off-by: Tibor Benke <ihrwein@gmail.com>
afunix: Remove socket acquire support.

This functionality goes into systemd-source.

Signed-off-by: Tibor Benke <ihrwein@gmail.com>
systemd: Fix config compilation to support systemd source.

Signed-off-by: Tibor Benke <ihrwein@gmail.com>
systemd: Make systemd source not fail without socket activation.

Signed-off-by: Tibor Benke <ihrwein@gmail.com>