Retire IPFIREWALL_NAT64_DIRECT_OUTPUT kernel option. And add ability
to switch the output method in run-time. Also document some sysctl
variables that can by changed for NAT64 module.

NAT64 had compile time option IPFIREWALL_NAT64_DIRECT_OUTPUT to use
if_output directly from nat64 module. By default is used netisr based
output method. Now both methods can be used, but they require different
handling by rules.

Obtained from: Yandex LLC
MFC after: 3 weeks
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D16647