Allow whitelisting unrecognized __Foo attributes with user_attributes= in .hhconfig

Revert of https://github.com/facebook/hhvm/commit/ec1031e25c189a12e0e89eb151c796da0b8fcdb2 ?

Potentially more useful for third-party code than FB - eg:
- vendor/foo/ uses a new user attribute from 3.7 which doesn't have runtime effects
- however, the main project keeps supporting (and being developed on) older versions

Otherwise, this is potentially an artificial restriction for library compatibility/feature adoption.

Some future attributes may have runtime effects, but leaving that to the user seems reasonable.

cc @elgenie