Fix the child's return path from fork so that fork will return 0

Fix the child's return path from fork so that fork will return 0
in the child.  This corrects a problem where linux/alpha binaries see
the child's return value of fork as the parent's pid.  This happens because
linux/alpha binaries apparently check the return value directly, rather
than looking for a non-zero value in a4, as *BSD & OSF/1 do.

Reviewed by:dfr@nlsystems.com