Some minor code cleanup inspired by clang -Weverything
Some general code cleanup

Make the stubs return failure, not success.
Add more comments.
Simplify bit-checking conditionals.
Patch a couple potential memory leaks.
Unify some code between preprocessor-conditional branches.
Be more compliant with RFC 2744

The handling of the output_token of GSS_Accept_sec_context()
is pretty poorly thought-out, as specified.