It appears that if you kick a subprocess using spawn()-family with spawn/nowait
or spawn/detach, there's no way to get the info of child process pid.  The spec
says they return unspecified exact integer.  Besides, on Windows you need
a process handle instead of pid (although this is POSIX srfi, swpan-procedures
do mention Windows).

Since we already have process objects, how about lettting spawn-famiily
to return a process object instead of an integer?  To get the exit status,
you can apply wait on it.