Noticed by Julie DeWandel <jdewand@redhat.com>.

do_fork() needs to return the pid (or error), not the pointer to the
resulting process structure.  The process structure may not even be
valid any more, since do_fork() has already woken the process up (and as
a result it might already have done its thing and gone away).

Besides, doing it this way cleans up the users, which all really just
wanted the pid or error number _anyway_.

This fixes the x86 users, other architectures need to be fixed up as
well.