Improve type safety of SPNEGO helper functions

Improve type safety of SPNEGO helper functions

Modify init_ctx_new(), init_ctx_cont(), acc_ctx_hints(),
acc_ctx_new(), and acc_ctx_cont() to operate on SPNEGO context handles
rather than generic GSS context handles.  For the functions which
create a new context, move the context parameter to the end and follow
the usual practices for output parameters.  Adjust stage 1 flow
control in spnego_gss_accept_sec_context() to allow acc_ctx_new() to
always be a context constructor.