Add dynamic reverse tunnel

[koziolku]

SSH allows estabilishing reverse dynamic tunnels using switch -R [host:]port. The usability is limited by default to localhost, until the server administrator decides to add an appropriate option in its' configuration (for example, GatewayPorts on in sshd_conf, but it is helpful anyway, even if not commonly used.

My idea from the interface is to allow users to define reverse dynamic tunnels by adding the REMOTE DYNAMIC option in drop-down menu when defining new tunnel, and create a setup opposite to DYNAMIC option, by requiring the Port and allowing the IIP/hostname to be filled, but in Remote (Connect to) area, instead of Local (Listen). It may be useful to change DYNAMIC option into LOCAL DYNAMIC to avoid confusion, maybe with some other grammar changes.

All of the changes, of course, should be connected to proper commandline switches or other mechanisms used in the program. It would allow the remote party to make SOCKS-based TCP connections to our network, if needed. Would be super useful to have complete set of tunnelling functionalities.