next version of OAP patch

Hi openbsc, here is my improved version of the OAP aka SGSN-ID patch series. This series is pushed to the neels/sgsn-id-3 branch. This version is substantially simpler, because I decided against factoring out the IPA client. Instead, the gprs_gsup_client.c will directly call the OAP API to handle OAP messages. Factoring out the IPA client was not a requirement and taking too much time, and since the name will remain "GSUP client" (to avoid changing the VTY configuration names among other things), it does after all make sense to think of it as: "the GSUP client employs OAP to authenticate" instead of "GSUP and OAP are two separate clients on the IPA wire". The OAP spec has changed. Particularly instead of SRES and Kc, the challenge response now sends XRES only; and the spec now includes an AUTS request (not yet implemented). Going through the changes, I've made a few fixes (like wrong file header comments), and made some changes. The OAP API looks quite different, and now merely returns struct msgb* and lets the caller take care of sending. This time, OAP has its own separate testsuite tests/oap. I have further OAP tests in mind to add; particularly the complete SGSN registration test from the previous patch version is not yet included here. Anyway, the OAP API test has become more comprehensive. Some of the patch mods have come from comments by hfreyther and jerlbeck, other changes were obvious from reviewing the code. I think this time around the patch is much more mature. I hope you agree, if not I'll gladly improve it further, of course :) Thanks for your reviews! ~Neels