process change notification prototype

[Justus Winter]

Hi :)

this is an early patch series implementing process notifications in
the proc server.

[PATCH 1/7] proc: remove unused file exc-reply.defs
[PATCH 2/7] proc: Remove unused declaration of zombie_list

Cleanups that can be merged independently of the rest.

[PATCH 3/7] libihash: add HURD_IHASH_ITERATE_ITEMS macro

I'd also like to propose this as an alternative to the
HURD_IHASH_ITERATE macro that only exposes the value, not the
key. This one gives one both the key and the value. The implementation
is also a lot less scary.

[PATCH 4/7] libnotify: add a general notification library

This is my shot at a hurdish general purpose notification
library. Please comment.

This could also be used in /hurd/init for the shutdown
notifications, and Richard suggested that we could have another class
of shutdown notifications available for non-privileged users that may
not stall the shutdown process (otoh I'm not so sure anymore that the
current mechanism can stall it, but I think this happened to me
once).

The function hurd_notify_do_notify_wait is not tested, the idea is to
asynchronously notify all receivers and then collect the responses.

[PATCH 5/7] hurd: add notification callbacks for the process
[PATCH 6/7] proc: implement proc_notify_new_processes
[PATCH 7/7] hurd: add proc_notify_new_processes

With these in place my cgroupfs can track all processes that play by
the book, i.e. that do use proc_child to mark a new process as their
child.

Cheers,
Justus