tests/netinet: add few tests for sending a broadcast packet

1. Set SO_BROADCAST and send to INADDR_BROADCAST.
2. Set SO_BROADCAST, set IP_ONESBCAST and cycle through broadcast capable interfaces sending a datagram on each.
3. Set SO_BROADCAST and send to local broadcast address.

For each test we bind a second socket to check whether the broadcast
is actually received.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49040