Use the HTTP Last-Modified http header as the mtime value for ADD cmd when present

Closes #8331

Signed-off-by: Doug Davis <dug@us.ibm.com>
hmm. I get the feeling we can probably do this without `syscall` and instead use `time` all the way through. This would (if not more efficient) be more idiomatic. Can we adjust this for that behavior or is there a specific reason you did it this way?