socket: introduce SELinuxLabelViaNet option

This makes possible to spawn service instances triggered by socket with
MLS/MCS SELinux labels which are created based on information provided by
connected peer.

Implementation of label_get_child_label derived from xinetd.

Reviewed-by: Paul Moore <pmoore@redhat.com>
networkd: don't consider deprecated or tentative addresses when determining operstate

https://bugs.freedesktop.org/show_bug.cgi?id=81287