Changes

linux: factorize some fd/path reading functions back

There's no strong need to have read_fd_as_cpumask/list()
separated from read_path_as_cpumask/list(), merge them back.

Also it avoids using open() in some cases (no fsroot) and
hwloc_open() in others (fsroot).

Also it will simplify support for readfile() (if it ever gets merged)
since we now open/read/close in the same function, but there's still
some work to do for reading cpumask/cpulist with readfile().

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

linux: rework read_path_by_length() return value

Return the length read instead of success/failure.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

linux: use helpers for reading tid location

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

linux: use helpers for reading nvidia gpu numa nodes

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

linux: use helpers for reading os device numa node

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

linux: use helpers for reading the pci config space

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

tests/diff: fix the test when the first NUMA node is CPU less

NUMA differences may actually appear before PU differences if
the first NUMA node is CPU less.
Handle both orders in the assertions.

Thanks to James D. Trotter for reporting the issue.
Closes #374.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>