Change lowest address on subnet (host 0) not to broadcast by default.

The address with a host part of all zeros was used as a broadcast long
ago, but the default has been all ones since 4.3BSD and RFC1122. Until
now, we would broadcast the host zero address as well as the configured
address. Change to not broadcasting that address by default, but add a
sysctl (net.inet.ip.broadcast_lowest) to re-enable it. Note that the
correct way to use the zero address for broadcast would be to configure
it as the broadcast address for the network.

See https:/datatracker.ietf.org/doc/draft-schoen-intarea-lowest-address/
and the discussion in https://reviews.freebsd.org/D19316. Note, Linux
now implements this.

Reviewed by: rgrimes, tuexen; melifaro (previous version)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D31861

(cherry picked from commit fd0765933c3ccb059ad7456e657b2e8ed22f58b0)