sd-daemon: Set SO_LINGER on AF_VSOCK notify socket fds

sd-daemon: Set SO_LINGER on AF_VSOCK notify socket fds

We're still seeing problems in mkosi CI where we lose notify messages
sent over vsock from virtual machines because the virtual machine shuts
down before the socket buffers are flushed.

Let's try setting SO_LINGER on vsock sockets so that the kernel waits
until the buffers are flushed when we close the file descriptor.

To make sure it works, let's get rid of our previous workaround that
sends EXIT_STATUS earlier to reduce the chance of it not arriving.
This way, if it doesn't work, we'll notice it immediately due to CI
failures.