Allow multiple simultaneous sessions

[rpwoodbu]

This will likely require a major UI change (which needs to happen anyway, as it is currently hideous), but also some changes to the code to allow multiple "instances". Some work toward this has already been done with the GetPOSIX() call; it should be able to distinguish which instance to use by looking at the current thread ID.

[rpwoodbu]

Implemented in SHA 7a00275.

It turned out I didn't need a major UI change, not any changes to the NaCl module. But I'm not perfectly satisfied with the UI, either. The initial session setup window stays loaded all the time, just minimized when you connect. Go back to it to start another session. If you close that window, it may be difficult to get it back without closing all sessions. On Linux, I find that I can just go back to the "Apps" page and launch it again. But on Chrome OS, clicking on the app in the Apps "drawer" (not the dock) just puts the latest Mosh window into focus, without calling onLaunched().

Furthermore, I cannot find a way to prevent closing of a window by accident. This seems to be a purposeful design decision for Chrome Packaged Apps. So that won't do it.

Anyway, perfect is the enemy of good, so I'm calling this done.