Fix dropfile support for non-privileged salt-master user.
Fix dropfile support for non-privileged salt-master user.

In 02d9684, a check was added to ensure that the crypto dfn file was
made writable on windows before overwriting it.  However, read-only
files are not only read-only on windows, they are read-only for
non-privileged users as well.

If you run the salt-master as a non-privileged user, you would observe
that the first child -- running _clear_old_jobs -- disappeared (went
defunct/zombie) after a while.  This patch fixes the cause of the
problem.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt