Rename llc_data_req_handler with llc_build_and_send_pkt, following my
plan to have LLC look more like TCP/IP and to slowly remove all the ugly
prim types and sap->{req,ind,conf}.

No problems with Appletalk up to now as it only uses UI and I'm up to
now only concentrating on connection mode, so that we can remove all
the duplicated work in core and PF_LLC.