Implement flist properly on GNU Hurd kernel

[lenticularis39]

Currently, only find is available for flist generation on GNU Hurd kernels. Implement either working fl_wrapper, or a unionmount translator with the same function, to get proper flist generation without capturing operating system noise and other changes.

[lenticularis39]

flist was segfaulting because on Hurd, PID 1's parent is itself, and that case was not handled. That is fixed now, but there are still some issues:

Failed to get atpath: 0, Makefile.osrbiD1
Failed to get atpath: 0, Makefile
Failed to get atpath: 0, Makefile.orig
Failed to get atpath: 0, Makefile
...
Created file outside allowed top-level dir: dev/null
Created file outside allowed top-level dir: dev/nul

[lenticularis39]

I disabled atpath() login on GNU Hurd since it is by design not possible without implementing tracking of open() directly in the wrapper, which is currently not done. Linux does atpath tracking inside VFS and gives access to it via readlink(), Hurd does no such thing.