pfkey: Deprecate pfkey

pfkey: Deprecate pfkey

The pfkey user configuration interface was replaced by the netlink
user configuration interface more than a decade ago. In between
all maintained IKE implementations moved to the netlink interface.
So let config NET_KEY default to no in Kconfig. The pfkey code
will be removed in a second step.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Antony Antony <antony.antony@secunet.com>
Acked-by: Tobias Brunner <tobias@strongswan.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Tuomo Soini <tis@foobar.fi>
Acked-by: Paul Wouters <paul@nohats.ca>