Make devfs cloning a sysctl/tunable which defaults to on.
If devfs cloning is enabled, only the super-user may create
tun(4)/tap(4)/vmnet(4) instances. Devfs cloning is still enabled by
default; it may be disabled from the loader or via sysctl with
"net.link.tap.devfs_cloning" and "net.link.tun.devfs_cloning".
Disabling its use affects potentially all tun(4)/tap(4) consumers
including OpenSSH, OpenVPN and VMware.
PR: 105228 (potentially also 90413, 105570)
Submitted by: Landon Fuller
Tested by: Andrej Tobola
Approved by: core (rwatson)
MFC after: 4 weeks
cd83bbd2aa