srm: Treat TReturnStatus as immutable

Replaces most TReturnStatus instantiations to treat the object as
immutable. Since TReturnStatus is auto-generated, it isn't possible
to actually make it immutable, but our code becomes cleaner if we
treat it as if it was.

I deliberately didn't change any of the status messages, except when
it was obviously wrong. Subsequent patches will focus on the status
messages.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5997/