bridge: Set member_ifaddrs=1 by default

[ivy]

Summary

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.

Keep the default value as 0 in main, but set it to 1 for stable/15 so
users and developers have advance warning to migrate.

This is a direct commit to stable/15.

[jrtc27]

Please commit this to main, it’s not appropriate to leave that broken

[markj]

In D52336#1194967, @jrtc27 wrote:

Please commit this to main, it’s not appropriate to leave that broken

I tend to agree. Even on main this change is pretty disruptive. For now I just set this sysctl manually since it's not very easy to work around some of the fallout related to DHCP.

[ivy]

based on feedback, i will land this in main unless anyone objects. however, it depends on D52335 which no one has reviewed yet.