network: NetLabel integration

network: NetLabel integration

New directive `NetLabel=` provides a method for integrating static and dynamic
network configuration into Linux NetLabel subsystem rules, used by Linux
Security Modules (LSMs) for network access control. The label, with suitable
LSM rules, can be used to control connectivity of (for example) a service with
peers in the local network. At least with SELinux, only the ingress can be
controlled but not egress. The benefit of using this setting is that it may be
possible to apply interface independent part of NetLabel configuration at very
early stage of system boot sequence, at the time when the network interfaces
are not available yet, with netlabelctl(8), and the per-interface configuration
with systemd-networkd once the interfaces appear later.  Currently this feature
is only implemented for SELinux.

The option expects a single NetLabel label. The label must conform to lexical
restrictions of LSM labels. When an interface is configured with IP addresses,
the addresses and subnetwork masks will be appended to the NetLabel Fallback
Peer Labeling rules. They will be removed when the interface is
deconfigured. Failures to manage the labels will be ignored.

Example:
```
[DHCPv4]
NetLabel=system_u:object_r:localnet_peer_t:s0
```

With the above rules for interface `eth0`, when the interface is configured with
an IPv4 address of 10.0.0.123/8, `systemd-networkd` performs the equivalent of
`netlabelctl` operation

```
$ sudo netlabelctl unlbl add interface eth0 address:10.0.0.0/8 label:system_u:object_r:localnet_peer_t:s0
```

Result:
```
$ sudo netlabelctl -p unlbl list
...
 interface: eth0
   address: 10.0.0.0/8
    label: "system_u:object_r:localnet_peer_t:s0"
...
```