Telnet SRP and encryption

I don't know how many clients support this, but it would be interesting to eventually add.

Telnet SRP is described in: https://tools.ietf.org/html/rfc2946 and https://tools.ietf.org/html/rfc2944

SRP: https://en.wikipedia.org/wiki/Secure_Remote_Password_protocol

Other relevant RFCs:
- https://tools.ietf.org/html/rfc2942
- https://tools.ietf.org/html/rfc2943
- https://tools.ietf.org/html/rfc2945
