Extended SSH Support

[davidrg]

Built in SSH support was reintroduced with #27 but there is much more work to do to finish the job!

• Implement remaining "set ssh" commands that libssh can support #75
• Support using the various "SET TCP" settings for configuring the SSH socket #357
• Keyboard interactive authentication doesn't work #71
• The /subsystem qualifier doesn't work with kermit as a subsystem #72
• X11 forwarding #73
• Local and remote port forwarding #74
• Add new key exchange methods setting #82
• support ssh username@host syntax rather than forcing users to use the unfamiliar ssh host /user:username #81
• Implement ssh key functions #83
• Clear out all obsolete SSH commands that will ever be re-implemented #96
• Better SSH Agent support #105
• Support SSH over http proxies #78
• Support SSH through SOCKS proxy #361
• Dynamic port forwarding over SSH #297
• Support sending environment variables
• Send backspace key setting to the server (if possible)
• Support for running xauth. This is something K95 1.1.x/2.x never did, but it would be nice if set ssh xauth-location actually did something for once