ifconfig greX create disables IPv6 forwarding

[Daniel Corbe]

For some reason, every time I create a GRE interface on a FreeBSD IPv6
gateway, net.inet6.ip6.forwarding is disabled.  As long as I manually
re-enable it with sysctl, both the GRE tunnel and the IPv6 network
behind this machine will continue to work; however, it's certainly far
from ideal.

There's nothing in the log to indicate exactly what's going on here:

Feb  9 11:23:08 router rtadvd[27251]: interface added (idx=8)
Feb  9 11:23:08 router devd: Executing '/etc/pccard_ether gre0 start'
Feb  9 11:23:21 router rtadvd[27251]: non-zero lifetime RA but net.inet6.ip6.forwarding=0.  Ignored.
Feb  9 11:23:53 router last message repeated 2 times

Nor is there anything about it in dmesg.

# uname -a
FreeBSD router.corbe.net 10.1-RELEASE FreeBSD 10.1-RELEASE #0 r274401: Tue Nov 11 21:02:49 UTC 2014     root at releng1.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64

-DAniel