Support new format for SSH private keys

[Soulou]

ssh-keygen on some system seems to generate OPENSSH PRIVATE KEY instead of RSA PRIVATE KEY in the private key, we need to add the compatibility.

Some references:

hierynomus/sshj#276
duplicati/duplicati#3360

[dkocher]

This is as of OpenSSH 7.8 the default.

• 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.

Workaround is to use ssh-keygen -m PEM.

[dkocher]

The failure is a message like Bad item length: -1512035316 which is caused from reading the key in OpenSSHKeyV1KeyFile.

[EtienneM]

Stack trace is:

db/tunnel.go:61: fail to connect to SSH server
net/ssh/client.go:37:
crypto/sshkeys/read.go:55: Invalid SSH key or password: ssh: unhandled key type