When the P flag is set (i.e. Overwrite regular files before deleting them),

When the P flag is set (i.e. Overwrite regular files before deleting them),
do only unlink the file if we could indeed overwrite the file.
Old behaviour: rm -P /tmp/foo (foo mode 0444) would NOT overwrite foo,
but still delete it (with a warning: rm: foo: Permission denied)
New behaviour: Just the EPERM warning, but no deletion

Reviewed by:	bde