pci-arbiter: Prevent mapping IO region files

[Joan Lledó]

Hello,

Time ago I sent some patches to implement mapping region and ROM files using
mmap(). However, a BAR region can represent either memory or I/O space, and
only the former should be allowed to be mapped, since I/O BARs don't contain
physical memory addresses, but I/O addresses. I attached a small patch to
prevent mapping I/O region files.

On the other hand, in the past we discussed how to make IO spaces available
for users through the arbiter [1]. It seems the way to go is adding a new RPC
that checks for permissions, calls i386_io_perm_create() and returns the
resulting port. I could work on that. After all changes in the arbiter and the 
Hurd these last years, would still be useful to have such RPC?

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[1] https://lists.gnu.org/archive/html/bug-hurd/2017-12/msg00048.html