odhcpd: Implement RFC9762 DHCPv6 PD Preferred flag for PIOs

odhcpd: Implement RFC9762 DHCPv6 PD Preferred flag for PIOs

The RFC defines the P flag for use within PIOs, and defines its meaning
as "purely a positive indicator, telling nodes that DHCPv6 PD is
available and the network prefers that nodes use it".

The RFC abstract lays out:

The (P) flag is used to indicate that the network prefers that clients
use the deployment model in RFC 9663 instead of using individual
addresses in the on-link prefix assigned using SLAAC, or DHCPv6 address
assignment.

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/301
Signed-off-by: Álvaro Fernández Rojas <[email protected]>