setting net.link.bridge.member_ifaddrs=0 by default

hello,

since commit 0a1294f6c610948d7447ae276df74a6d5269b62e we have a new
sysctl, net.link.bridge.member_ifaddrs.  when set to 0, this has the
following effects:

- an interface with an IP address cannot be added to a bridge

- an interface inside a bridge cannot be configured with an IP address

- the system will not consider Ethernet frames addressed to link-level
  addresses of members to be local traffic

i would like to propose that we set this to 0 by default in 15.0-RELEASE.

my reasoning for this is that having bridge members with an IP address
assigned does not work, and has never worked, and it's clear from places
where users ask for support (such as IRC) that many people run into this
problem and do not understand what they're doing wrong, until someone
explains that they should not assign an IP address to bridge members.

so, enabling this by default has clear benefits for usability (and also
performance) as well as code simplification later when we can remove the
relevant code, perhaps in 16.0R.

thoughts?