Fix a lot of issues with smbpool or smbspool_krb5_wrapper as a cups backend

I got several reports that printing with smbspool as a CUPS backend is broken since Samba 4.8. Together with Andrej who is doing Samba QA we finally fixed all issues. Andrej wrote test case for our internal testing infrastructure and I started to fix all those bugs we discovered.

I ended up rewriting larger parts to get useful output do debug it as a cups backend. We have NTSTATUS return code and get better messages if something is wrong now.

Parts of this is already covered by samba3.blackbox.smbspool but we can't test everything in selftest.