Demystify catopen return value in fstrace

Demystify catopen return value in fstrace

For some reason, the code was, on all platforms other than Digital
UNIX, assigning the return value of catopen to an integer, with
and then immediately assigning it to an nl_catd (the actual return
value type).  Remove all of that and store the return value directly
in an nl_catd variable.

Reviewed-on: http://gerrit.openafs.org/93
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Verified-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>