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

Move common/up_signal_dispatch.c to armv6-m, armv7-m, and armv7-a.  The armv7-a version needs to be different to handle the case where we are dispatch kernel mode signals when running under a user mode group