vfs: use 64bit for file offset and size

Fixes #1246
vbox: enable auto test for 32bit Genode host

Issue #1246
run: disable network tests on rpi

No working network driver so far.
lx_fs: improve root directory handling

- correctly catch and report non-existing root directories
- remove *all* leading slashes from root-directory attributes and
  sanitize empty declarations to current working directory