This is a draft version of the code for #51
Improved credential checks in the GeniStdMA and GeniStdSA (the "Federation MA/SA APIs").
Instead of lots of copy-past reuse, most moethods now call GeniStd::RequireAuthorizationForSlice and GeniStd::RequireUserAuthorization
GeniStd::RequireAuthorizationForSlice should now support multiple credentials, finding the needed slice credential in the list.
This is completely untested. I'll get back to this when I have tested it.