networkd-network: Introduce DHCPv6 PD knob for RFC 7084 WPD-4

networkd-network: Introduce DHCPv6 PD knob for RFC 7084 WPD-4

RFC 7084, WPD-4, requires Customer Edge end routers to behave
according to the following:

"WPD-4:  By default, the IPv6 CE router MUST initiate DHCPv6 prefix
           delegation when either the M or O flags are set to 1 in a
           received Router Advertisement (RA) message.  Behavior of the
           CE router to use DHCPv6 prefix delegation when the CE router
           has not received any RA or received an RA with the M and the
           O bits set to zero is out of scope for this document."

Since it cannot be automatically detected whether DHCPv6 is to be
operated as an CE end router or whether to initiate an Informational
exchange to obtain other useful network information via DHCPv6 when the
Router Advertisement 'O' bit is set, a 'ForceDHCPv6PDOtherInformation'
boolean network configuration option in the '[DHCP]' section of a is
introduced. Setting this option causes DHCPv6 to be started in stateful
mode, although only the 'O' bit is seen in the Router Advertisement.

When 'ForceDHCPv6PDOtherInformation' is set and the Router Advertisement
has only the Other information 'O' bit set, disable requests for IA_NA
addresses.

Fixes #9745.