* libguile/posix-w32.c: Include gc.h and threads.h.
(proc_record): New structure tag.
<procs, proc_size>: New static variables.
(find_proc, proc_handle, record_proc, delete_proc): New utility
functions.
(start_child): Return value is now pid_t, as it is on Posix
platforms. Record the new process and returns its PID, instead of
returning a handle. Fix the recursive call.
(waitpid, kill, getpriority, setpriority, sched_getaffinity)
(sched_setaffinity): Look up the PID in the recorded subprocesses
before trying to open a process that is not our subprocess. Make
sure any open handle is closed before returning, unless it's our
subprocess.
* libguile/posix-w32.h:
* libguile/posix-w32.c (kill, waitpid, getpriority, setpriority)
(sched_getaffinity, sched_setaffinity): Add MinGW implementations.
Also, provides macros that on Posix hosts are in sys/wait.h, like
WIFEXITED and WTERMSIG.
(start_child): Add implementation.
