utils.py: Add save_file_atomic() helper

This is a context manager that can be used to divert file writes into a
temporary file, which is then renamed into place once writing is
complete. It is primarily intended for use by source plugins which
download files, so they can ensure that their downloads appear atomic
and there is no risk of leaving half-downloaded files in the cache.

So far this is not used in the core, but it is needed by the Docker
source plugin that is proposed for the bst-external plugins repo. See:
bst-external!9