Fixing issue #258, bad keyfile permissions.

This spits out a warning to the console, but ideally it would also
issue a warning to the GUI for those who didn't start it from the
console. N.B. the warning is a one shot thing, since it fixes the
problem in a way essentially undetectable in the future, so it
should be done right if it is to be done at all.

Maybe we should even disable all keys automatically if the keyfile
is found in an insecure state.