Improve internal API for GSS sequence numbers

Improve internal API for GSS sequence numbers

Use an opaque structure type instead of a void pointer for the
sequence number state.  Rename all functions to use a g_seqstate
prefix rather than a mix of g_order and g_queue.  Remove the
unneccessary indirection from the state object parameter in
g_seqstate_check and g_seqstate_free.  Return OM_uint32 where we
return a GSS major code, long where we return an errno value, and void
where we can't fail.