CA-82314: networkd: introduce Linklocal6 IPv6 address mode

This commit redefines the IPv6 address mode "None6" to mean "no IPv6 address at
all, not even a link local one", and introduces the new mode Linklocal6, which
means "only IPv6 address, except a link local one".

Previously, it was not possible to get networkd to remove the link-local IPv6
address from an interface.

When a link local IPv6 address is requested, it is derived from the interface's
MAC address as commonly done.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
CA-82314: nm: Allow (only) bridge interfaces to have a link-local IPv6 address

Due to a recent change in xcp-network, the None6 IPv6 address mode removes all
IPv6 addresses, including the link local one.  To avoid problems with bonding,
we only want bridge interfaces to have any IP addresses, while the physical
interfaces may not even have a link-local address.

This commit ensures that bridge interfaces are configured accordingly.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1447 from robhoes/ca82314

CA-82314: remove link-local IPv6 addresses from physical interfaces