os::copy_file needs to copy the execution bit

[erickt]

Cargo briefly used os::copy_file, but switched back to shelling out to cp in order to preserve the execution bit (See #2456). We should update os::copy_file to add this functionality.

[graydon]

non-critical for 0.6, de-milestoning

[catamorphism]

I fixed this a while ago; see the test os::tests::copy_file_ok.