dhcp: systemd: support DHCPv6 prefix delegation

Add support to the internal DHCP client for requesting a prefix and
distributing it to interfaces with 'shared' IPv6 mode.

The systemd-networkd API currently allows to request only a single
prefix and so there will be issues when the number of downstream
interfaces is greater than the number of /64 subnets available in the
returned prefix; but this is still an improvement over the previous
situation when no prefix was requested at all.

#247
18 jobs for bg/dhcp-pd-internal in 47 minutes and 10 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #666535
allowed to fail
checkpatch

00:01:30

manual #666541
allowed to fail manual
t_centos:7.5.1804
passed #666542
t_centos:7.6.1810

00:25:51

passed #666548
t_debian:10

00:20:25

passed #666547
t_debian:9

00:20:10

passed #666550
t_debian:sid

00:20:33

manual #666549
allowed to fail manual
t_debian:testing
manual #666536
allowed to fail manual
t_fedora:28
passed #666537
t_fedora:29

00:47:09

passed #666538
t_fedora:30

00:38:39

passed #666539
t_fedora:31

00:35:02

manual #666540
allowed to fail manual
t_fedora:rawhide
passed #666543
t_ubuntu:16.04

00:21:03

passed #666544
t_ubuntu:18.04

00:21:20

manual #666546
allowed to fail manual
t_ubuntu:devel
manual #666545
allowed to fail manual
t_ubuntu:rolling
 
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/756/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/756/

05:57:57