Create secret files with private modes

[benthecarman]

Previously our api key file and TLS key were both written with normal file permissions and then we later updated it 0o400. Now we make sure we create the file with 0o400 permissions so we never have a lapse where the file might be readable by someone we don't want.

[ldk-reviews-bot]

👋 Thanks for assigning @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.