These patches fill out shepherd's procedures for running processes to
completion.  They add a replacement for 'system' to complement the
existing replacement for 'system*', and add a 'fork+exec+wait-process'
procedure so that the flexibility of that family of procedures is
available for this use case as well.  It also improves error handling in
the event that an exception occurs while spawning a process in the
process monitor, which would normally kill that essential fiber.