To avoid user and automation confusion, prefix the temporary files with
a ".". That way, if something scans the directory for files, it's less
likely to encounter one of our temporary files.
This will become very relevant in subsequent patches.
This patch adds a new internal/safeio package, which is meant to implement
safer version of some I/O related functions.
For now, only an atomic version of ioutil.WriteFile is implemented. More may
be added later if there's a need for them.
