protected payloads for GNU Mach

[Justus Winter]

Hi :)

this patch series implements protected payloads on top of the mach
messaging system.  It preserves the semantics for programs not using
this functionality.

This is the kernel part of my work that will (hopefully) speed up the
Hurd.  For a detailed sketch see:

id:20131128235647.1092.90189@thinkbox.jade-hamburg.de
or
http://marc.info/?l=debian-hurd&m=138568308704140&w=2

I believe that the kernel part is feature complete.  I had this patch
series in the gnumach packages in hurd-ci since January 18th.  I have
not noticed any problems that I'd attribute to this series, hence I
strongly believe that this does in fact preserve the current semantics
for existing programs.  I did not change the implementation since I
posted it in the mail thread mentioned above, I just added
documentation.

Cheers,
Justus