Expose 'handle' as well as 'id' on LLProcess objects.

On Posix, these and the corresponding getProcessID()/getProcessHandle()
accessors produce the same pid_t value; but on Windows, it's useful to
distinguish an int-like 'id' useful to human log readers versus an opaque
'handle' for passing to platform-specific API functions. So make the
distinction in a platform-independent way.