Make BPF backend non-experimental?

As one of the people who has worked with and reviewed the BPF port I'm in
favor of this move.

Reasons:

1) Alexei has been very active on the port, fixing problems unprompted
2) As one of the people who've had to had on it in the last few months it
wasn't an issue to keep up to date with the other ports.

Unless there are objections by EOD Monday I feel confident making the call
here and having Alexei add it to the list of official targets.

Thanks Eric!
Bumping the thread one more time.