The previous default was to expect passphrases to be ASCII or
rather Latin-1. It would be reasonable to expect modern keys to
use UTF-8 instead.
This is a non-breaking change if passphrases only use characters
that fall within ASCII.
Fixes#2102
AES-256 uses a 32-byte (256-bit) key size. This un-breaks the loader and
tests added for AES-256-CBC and AES-256-CTR PEM keys.
* OpenSSHKey: correctly parse encrypted PEM AES-256-CBC/AES-256-CTR keys
* OpenSSHKey: use correct key derivation for AES-256
