upstream: support PKCS8 as an optional format for storage of

upstream: support PKCS8 as an optional format for storage of

private keys, enabled via "ssh-keygen -m PKCS8" on operations that save
private keys to disk.

The OpenSSH native key format remains the default, but PKCS8 is a
superior format to PEM if interoperability with non-OpenSSH software
is required, as it may use a less terrible KDF (IIRC PEM uses a single
round of MD5 as a KDF).

adapted from patch by Jakub Jelen via bz3013; ok markus

OpenBSD-Commit-ID: 027824e3bc0b1c243dc5188504526d73a55accb1