Set correct permissions on <dest> file creation

The creation of a new output file each time the config changes causes problems with its ownership and permissions; when the output file is on a Docker volume.
Without doing additional user adding/user space mapping stuff, Docker writes the file as root:root and 0600, which for all sense and purposes makes it unreadable for everything else.
If a file already exists, then the file's ownership and permissions are left untouched by Docker.

I couldn't see why a temp file was being used so this patch removes the use of it, and instead will truncate and overwrite the existing file.
Both of these calls ignore the error return value.