Add UDP encapsulation of ESP in IPv6

This patch provides UDP encapsulation of ESP packets over IPv6.
Ports the IPv4 code to IPv6 and adds support for IPv6 in udpencap.c
As required by the RFC and unlike in IPv4 encapsulation,
UDP checksums are calculated.

Co-authored-by: Aurelien Cazuc <aurelien.cazuc.external@stormshield.eu>
Sponsored-by: Stormshield
Sponsored-by: Wiktel
Sponsored-by: Klara, Inc.