Harden It's Dangerous key management.

Harden It's Dangerous key management.

The previous code was theoretically subject to timing attacks, where
an attacker could read the key in between the time it was saved to the
file and when the chmod happened.  This version prevents that by using
umasks to ensure the files always have the right permissions.

This version also avoids using a key that cannot be saved due to some
system setup bug.