Merge branch 'refs/heads/t/ssh'

A medium sized series which teaches both crypto backends about the
RFC4716 key format, which has become the default for ssh-keygen as
of openssh-7.8.

Was cooking for almost a year.

* refs/heads/t/ssh:
  manual: Instruct the user to create RFC4716 keys.
  openssl: Add support for RFC4716 keys
  openssl: Move get_private_key() down.
  gcrypt: Add support for RFC4716 private keys.
  crypt: Rename decoding functions.
  gcrypt: Introduce read_openssh_bignum().
  gcrypt: Factor out read_pem_rsa_params().
  gcrypt: Let read_bignum() return bits, not bytes.
  gcrypt: Let decode_key() return blob size through additional argument.
  gcrypt: Drop unnecessary arguments of decode_key().