Simulation: Change how simulated UART data availability is signaled.  The last change is not safe (but I don't like this workaround either -- maybe something better will come to me).

simulation: Add a simulatied UART input device.  Also lots of misc clean-up for stylistic compatibility with other architectures

More vfork():  If we get to vfork() via system call, then we need to clone some system call information so that the return form the cloned system call works correctly

Fix vfork().  Now that arguments are kept on the stack, the way that arguments are passed from parent to child in vfork() must change.  This bug has always been present, but was not visible with the old strdup() way of passing arguments