(apply_attributes): New function.

(apply_attributes): New function.
(copy_reg): Use apply_attributes instead of open-coding the pieces.
Now, failure to preserve file attributes does not cause mv to change
its exit status, and such failures elicit warning diagnostics.  This
behavior is required by POSIX.  Before, failure to preserve ownership
due to insufficient access was diagnosed only for root.
Prompted by a report from Bengt Martensson.