change writing strategy to avoid partial writes when interrupted

[scottwalters]

File writing was so delicate and careful that I hate to touch it, but there's only
one way to replace a file in an atomic fashion on Unix that I know of.

Twice now I've observed corrupted config files apparently resulting from the program being interrupted part way through re-writing the file. This changes strategy to use an atomic operation to replace the config file with a rename() after it is completely written.

[haarg]

The primary reason the existing implementation was chosen was to prevent the possibility of changing the file permissions or ownership. It might make sense to have the code attempt to preserve file ownership where possible.

Other than that I think this is a much better way to go.

[jalcine]

👍