So we can detect IPv6ND or DHCPv6 overflow as well as route(4)
overflow.

Apple Conformance testing now requires this.
However, if associated SSID changes on wireless networks,
any remembered address is reset.

Carrier UP/DOWN state is handled by media change events on BSD.
When the carrier state changes, it's always to LINK_STATE_DOWN or
LINK_STATE_UP.
If we receive LINK_STATE_UNKNOWN from RTM_IFINFO it means the
interface doesn't change media change events and as such won't
report carrier state changes. In this instance, we can only rely
on IFF_UP being set, which is the same condition dhcpcd needs
for LINK_STATE_UP.

This allows a secondary router to broadcast prefixes it controls
without being a default router itself.

This allows the same IPv6 address to exist on more than one
interface. Whenever dhcpcd address an IPv6 address, it will
advertise it along with the hardware address of the preferred
interface.

This is heavliy reliant on the kernel supporting this as it's the
kernel that handle the Duplicate Address Detection.
In a nutshell it needs to support RFC 7527 and ignore NA packets
from any hardware address the host owns.

Currently the only known kernel that fully supports this is
NetBSD-8.99.27