Add openssh.com symmetric cipher

[DoctorD90]

Hi! First of all, thanks! This is one of my best favourite/used app ;)
It's awesome and I really thank you for your great work!

I was updating my SSHd config; I hardened the server as much as possible.
Testing with ConnectBot I noticed that it doesn't support a couple of openssh' ciphers.
Would be possible to add them please? I refer to:

• chacha20-poly1305@openssh.com
• aes256-gcm@openssh.com
• aes128-gcm@openssh.com

Thank you!

[kruton]

This requires a restructuring of how the cipher part of this libary works. Currently it separates the cipher away from the MAC.