file_unmanaged_save_data()/file_unmanaged_move() are not atomic

The problem

file_unmanaged_save_data() calls file_unmanaged_move().

file_unmanaged_move() calls file_unmanaged_copy(), which calls cp.

cp is not atomic, mv is (per http://stackoverflow.com/questions/11231198/should-my-script-use-cp-or-m... and others).

This can and does lead to race conditions where one process starts writing a file, and another starts reading it before it's finished, and gets an incomplete/empty file.

At least three contrib projects have their own versions of file_unmanaged_save_data(), dating back to 2012 or earlier, but I couldn't find a core issue:

boost: http://api.drupal.psu.edu/api/drupal/modules!contrib!boost!boost.module/...
advagg: http://www.rit.edu/drupal/api/drupal/sites!all!modules!advagg!advagg.mis...
agrcache: #2628918: Address deployment race conditions

Proposed solution

1. fix file_unmanaged_save_data() to use rename() instead of copy()

If that's too much of a behaviour change, we could add a new function/method, but don't really see why it'd be a problem to just fix it.

Remaining tasks