* callproc.c (Fcall_process): Fix vfork portability problems.

* callproc.c (Fcall_process): Fix vfork portability problems.

Do not assume that fd[0], count, filefd, and save_environ survive
vfork.  Fix bug whereby wrong errno value could be reported for
pipe failure.  Some minor cleanups, too, as follows.  Move buf and
bufsize to the context where they're needed.  Change new_argv to
be of type char **, as this is more convenient and avoids casts.
(CALLPROC_BUFFER_SIZE_MIN, CALLPROC_BUFFER_SIZE_MAX):
Now local constants, not macros.