homed: explicitly set access mode of private/public signing key pair

homed: explicitly set access mode of private/public signing key pair

So far we relied that the temporary file logic would create the key
files with 0600 mode, but let's set the access mode explicitly:

1. Tighten private key file access from 0600 to 0400, after all we never
   want to write it again, it's not a mutable file.

2. Relaxed public key file access mode from 0600 to 0444, after all it's
   a public key file, and people should be able to see it if they want
   This is useful for propagating the key onto other systems if needed.