ssh-keygen(1): write OpenSSH format private keys by default
instead of using OpenSSL's PEM format. The OpenSSH format,
supported in OpenSSH releases since 2014 and described in the
PROTOCOL.key file in the source distribution, offers substantially
better protection against offline password guessing and supports
key comments in private keys. If necessary, it is possible to write
old PEM-style keys by adding "-m PEM" to ssh-keygen's arguments
when generating or updating a key.