Add Terminate dnode.Function to Exec type

[acbodine]

Koding user's need to be able to kill an exec'd process.

NOTE: This is not the same things as dettaching from a session in the front-end. In other words, if a user reloads their browser window, we should do what the current implementation does and leave the exec'd process in tact. This way a Koding user can re-attach to existing sessions(execs) that live inside any of the containers in a pod.

Make the dnode.Function interface for exec kite handler be the same as webterm.connect