add plan9 support

[Elbehery]

fixes #587

cc @ahrtr @fuweid @tjungblu

[fuweid]

I don't have plan9 env to verify it.

If we have to support multiple platforms, I think bbolt should consider use QEMU/virt for testing.
Without testing env, it's easy to break the change.

[Elbehery]

I dont have plan9 either :/

[ahrtr]

Thanks @Elbehery for the PR.

Just as I mentioned in #587 (comment), it isn't difficult to make code change, but we do not have real plan9 environment nor proper test suite (e.g. qemu, FYI. #577) to verify the PR.

Proposed actions:

• I expect someone who has real plan9 environment to contribute & verify the PR.
• Please anyone feel free to introduce qemu to verify all the existing platforms which are supported by bbolt.

Will close this PR for now.