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/