pam_ssh: Don't allow a bogus passphrase for unencrypted keys.

pam_ssh:  Don't allow a bogus passphrase for unencrypted keys.

key_load_private() ignores the passphrase argument if the private key
is unencrypted.  This defeats the nullok check, because it means a
non-null passphrase will successfully unlock the key.

To address this, try at first to load the key without a passphrase.
If this succeeds and the user provided a non-empty passphrase *or*
nullok is false, reject the key.

While I'm here: Load the ECDSA key if there is one.

Obtained-from:  FreeBSD 227757, 219426, & 226101