stream: Add basic stream service for systemd sockets

stream: Add basic stream service for systemd sockets

This allows systemd socket activation by passing URIs such as systemd://foo
to plugins such as VICI.

For example setting charon.plugins.vici.socket = systemd://vici, a
systemd socket file descriptor with the name "vici" will be picked up.

So these would be the corresponding unit options:

  [Socket]
  FileDescriptorName=vici
  Service=strongswan.service

  ListenStream=/run/charon.vici

The implementation currently is very basic and right now only the first
file descriptor for a particular identifier is picked up if there are
multiple socket units with the same FileDescriptorName.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Closes strongswan/strongswan#79.