pnfsmanager: Check file type before overwrite

When preparing a file for upload for the SRM, pnfsmanager fails to check the
file type of any existing file with the same path. The upload eventually fails
during srmPutDone because Chimera refuses to move the file, but this failure
ought to happen during srmPrepareToPut. This patch resolves this problem.

Target: trunk
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7407/
(cherry picked from commit 84c8587315c1f80012e422955403bfd9374c1b7f)