vnet.9: Remove references to non-existant macros and update how to virtualize sysctls

Approved by: bcr, ziaee
MFC after: 1 week
MFC to: stable/14, stable/15
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D52620

(cherry picked from commit 22886d3af0657dae3b547b21e612642bb47e0321)