GitLab

- Always overwrites the destination, if it is a file.
- Does not touch destination permissions when overwriting.
- On POSIX, preserves and follows destinations that are symlinks.
- On POSIX, file permissions for the destination is platform dependent.

Then document these in the header, since these details may be important
to the caller.

Change-Id: Ibd47fa95aac29a35d798e5c19d1da1a64df63754
Reviewed-on: https://chromium-review.googlesource.com/686001
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505836}