Protected payloads for the Hurd servers

[Justus Winter]

Hello,

these patches make libports use Gnumachs protected payloads to
optimize the receiver lookup.  In fact, we can now dispatch most
messages without acquiring a lock in libports.

The first patch is for Gnumachs type definitions.  It changes
`mach_port_t' (and reply ports) so that we can inject an
`intranpayload' function via a preprocessor macro.  This function maps
payloads to port names.  I originally had this in hurd_types.defs, but
I came to the conclusion that the proper place is really the kernel
definitions itself.

This concludes a ~14 months effort that I thought would take two
weeks.  Silly me.

Cheers,
Justus