bridge: Set member_ifaddrs=1 by default

We aren't ready to release with this set to 0: there's no solution for
running DHCP on a bridge from a devd hook, there are missing edge cases
like network boot with the boot interface in a bridge, and third-party
tools (e.g., vm-bhyve) still direct users to use the incorrect
configuration and/or don't support the correct configuration.

Based on popular vote, we'll set this back to 0 in both main and for
stable/15.

MFC after: 9 hours
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D52336