netinet6: allow disabling excess log messages

RFC 4443 specifies cases where certain packets, like those originating from
local-scope addresses destined outside of the scope shouldn't be forwarded.
The current practice is to drop them, send ICMPv6 message where appropriate,
and log the message:

cannot forward src fe80:10::426:82ff:fe36:1d8, dst 2001:db8:db8::10, nxt
58, rcvif vlan5, outif vlan2

At times the volume of such messages cat get very high. Let's allow local
admins to disable such messages on per vnet basis, keeping the current
default (log).

Reported by: zarychtam@plan-b.pwste.edu.pl
Reviewed by: zlei (previous version), pauamma (docs)
Differential Revision: https://reviews.freebsd.org/D38644