Add a new exec_exec_file_name RPC

[Justus Winter]

Hi :)

this is a rebased patch series created by Emilio Pozuelo Monfort.
I've slightly amended the first patch.

This patch series makes the fakeroot translator usable, more precisely
the execution of interpreted scripts within the fakeroot environment.
Both the patch series and my amendment have received considerable
testing in the Debian hurd package.

Cheers,
Justus