HPCC-12755 Rationalize secure workunit interface

Another step towards being able to provide plugin workunit functionality.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
It could (though that's not a new issue - there are other places where a potentially null value is passed in on this reference). The code in checkWuScopeSecAccess relies on being able to check that secUser is non-null to detect these cases.

But it's probably sensible to fix as part of this rationalization - I'll change it to a pointer.