main: Write output ignition files with mode 0600

[cgwalters]

Because the config is commonly expected to have secret values, use mode 0600.
xref coreos/fedora-coreos-docs#306

[bgilbert]

Hmm, this introduces a divergence between files written with -o and those written by redirecting stdout. When running Butane from a container, we've generally recommended the latter. Does the divergence make things better or worse?