This implements the 'keep the initial thread around until every thread
in the group exits' concept in a different, less intrusive way, along
your suggestions.  There is no exit_done completion handling anymore,
freeing of the task is still done by wait4().  This has the following
side-effect: detached threads/processes can only be started within a
thread group, not in a standalone way.

(This also fixes the bugs introduced by the ->exit_done code, which made
it possible for a zombie task to be reactivated.)

I've introduced the p->group_leader pointer, which can/will be used for
other purposes in the future as well - since from now on the thread
group leader is always existent.  Right now it's used to notify the
parent of the thread group leader from the last non-leader thread that
exits [if the thread group leader is a zombie already].