copy-file: add error-code-returning variant.

copy-file: add error-code-returning variant.

* lib/copy-file.h (GL_COPY_ERR_*): New enumeration items.
(qcopy_file_preserving): New declaration.
* lib/copy-file.c (qcopy_file_preserving): Renamed from
copy_file_preserving. Change return type to 'int'. Don't emit an error
message here.
(copy_file_preserving): New function.
* tests/test-copy-file.c: Include <stdlib.h>.
(main): Test qcopy_file_preserving if the environment variable
NO_STDERR_OUTPUT is set.
* tests/test-copy-file-1.sh: Invoke test-copy-file.sh a second time,
with NO_STDERR_OUTPUT
* tests/test-copy-file-2.sh: Likewise.