Fix "netstat -gW" behavior broken in r259638.
netstat has two options for printing multicast tables:
sysctl (the default one for live systems) and kvm-based one (for cores).
It looks like kvm-based one hasn't been working since it's been introduced
in r190012 due to absence of mfctablesize kernel symbol.
Check for all ipv4-multicast symbols being correctly resolved was introduced
in r259638 regardless of 'live' value leading to "No IPv4 MROUTING" error
message.

Reported by:	Olivier Cochard-Labbé
MFC after:	1 week
Merge from CheriBSD:

commit 1d1b908107255ffdff4d17f015d8f057d73cc6cb
Author: Brooks Davis <brooks@one-eyed-alien.net>
Date:   Fri Mar 28 16:24:45 2014 +0000

    Add a long needed seatbelt.

    Exit with an error when make is called without a target at the top level
    rather than poluting the source tree and causing use confusion in future
    builds.

commit a9d9aa341b2f4308a227ab460ba85f1f287ad028
Author: Brooks Davis <brooks@one-eyed-alien.net>
Date:   Tue Apr 29 16:06:12 2014 +0000

    Simplify seatbelt added in 1d1b908 based in feedback.

    Discussed with:	imp@FreeBSD.org

Reviewed by:	imp
Sponsored by:	DARPA, AFRL