sftp: Keep permissions of copied file from remote

Alex Hermannrequested to merge
alexhe/kio-extras:sftp-keep-remote-perms into master

When copying from the remote to local, the initial copy unconditionally adds owner_write permission for some unspecified NFS issue.

Make sure that after copying the file, the permissions on the destination file become identical to that of the source file.

Copying from local to remote is not affected and is already correct.

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