Add a new PIO flag: DHCPv6-PD preferred flag.

The new P flag is defined in
https://datatracker.ietf.org/doc/draft-ietf-6man-pio-pflag/.

Bug: 315069348
Test: TH
Change-Id: Ie386cc8b9fbe136333b46c73da785e02983a582b
1 file changed