Some more refactor suggestions

[Pavel Fedin]

 Hello! I would like to suggest another code refactor, which could make the
code easier to maintain.
 This is a result of my study of spawn() vs fork() differences and ways to
implement runtime switching between spawn() and fork() in a simple way. The
idea is to unify child_execute_job() function.
 The unification can be done in the following way:
1. The function will always return child's PID.
2. On UNIX the function will call fork() in order to create a child by
itself.
3. job_fds[] and job_rfd can be handled in spawn() way in both cases.
Currently the differences are:
   a) fork() version calls fork(), then closes descriptors in the child
explicitly.
   b) spawn() version sets FD_CLOEXEC flags for these fd's, then calls
spawn(). This effectively means that these descriptors are not inherited by
the child. After spawn()ing FD_CLOEXEC flag is reset.
   I suggest that (b) approach can also be used on UNIX without bad effects.
We could FD_CLONEXEC for relevand fds, then call child_execute_job() which
does fork(). The effect should be the same as closing fd's in the child
manually. I suggest that parent process can even skip resetting this flag
since Make AFAIK never replaces itself with some other process which needs
to inherit all internal file descriptors. So, the code can be further
simplified.
 Any thoughts / pros / contras ?

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia