Allows to set advertised, optional, features attributes while sending greeting element.
Inside the 188.8.131.52, RFC 3080, The Greeting Message is defined that the greeting message support an optional attribute called "features". This attribute contains optional features that could be advertised to the remote peer.
This function allows to set the content of the "features" attribute. It will be used by any greeting generated. These features is only to be used to modify the channel management BEEP profile.
The function doesn't make any copy of the received value. You should not free passed in value.