Initiates a security context to send to a remote peer.
This wraps GSS_Init_sec_context according to RFC 2743
section 2.2.1.
The underlying mechanism might require several tokens to be
passed back and forth to establish the context. If
is_established returns zero after a call to this function
then the caller must wait for a token from the remote peer to
feed as remote_token in another call to this function.