SSH: add support for aes128-ctr and hmac-sha2-256

This cipher and hmac are RECOMMENDED as per RFC4344 and
RFC6668. RFC4344 notes that aes128-ctr is likely to be
promoted to REQUIRED in the near future.

Some distros of Linux and Illumos are already shipping with
aes128-cbc and 3des-cbc disabled by default due to security
concerns around chosen-plaintext attacks and other information
leaks. Without this patch, OTP SSH cannot connect to these
SSH servers in their default configuration.