PCI arbiter memory mapping

[Joan Lledó]

Hi Hurd,

I finally got memory mapping working in the pci arbiter. That means any 
user with the proper permissions can map the device region files 
generated by an arbiter. This is also working for nested arbiters.

For this I made changes in libpciaccess, gnumach, and the Hurd: 
pci-arbiter and libnetfs folders.

I got the code in some branches in different repositories. How should I 
share the code for your review? do you prefer diff patches by email or 
should I clean the branches and share the links?