KTLS: Enable KERN_TLS in GENERIC on amd64

Based on discussions on freebsd-arch@, enable KERN_TLS in
GENERIC on amd64, but leave it disabled via the
sysctl kern.ipc.tls.enable. Users wishing to enable
ktls must set kern.ipc.tls.enable=1

While here, fix wording in NOTES to mention that KERN_TLS
also does receive now.

Sponsored by: Netflix

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D28163