Add keystroke timing obfuscation to the client.

Add keystroke timing obfuscation to the client.

This attempts to hide inter-keystroke timings by sending interactive
traffic at fixed intervals (default: every 20ms) when there is only a
small amount of data being sent. It also sends fake "chaff" keystrokes
for a random interval after the last real keystroke. These are
controlled by a new ssh_config ObscureKeystrokeTiming keyword/

feedback/ok markus@