improve _exit()

[anttikantee]

Currently, _exit(2) causes the DomU to kick it. We could make it mimic normal semantics where only the application exits with a few simple improvements, since most of the resources used by the process are tracked by the rump kernel and will be released when the rump_lwproc exits.

• track application-side malloc's in per-process chains and release them at exit
• zero out per-application bss before process is started (for re-execution). re-init r/w data (how?)
• call exit_thread() instead of do_exit() (need to g/c other threads in multithreaded apps)

Something else? The above was "theoretically speaking" and off the top of my head.