Changes

distances: fix API comment indentation
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

distances: add a hidden id field before public distances
When we return a distances struct to the user, actually allocate a
larger container that points to the internal distances.
Might be used later to find the corresponding internal distances, for
instance if we ever need to refresh the public distances, or to
retrieve name, infos, etc.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

API: add hwloc_distances_release_remove()
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

core: add a name to internal distances structure
Not exported to users yet. Useful for better describing distances
structure, especially for upcoming matrices for objects with different
types.
Only "NUMALatency" is currently used, and that's not very useful since 
the distances structure type and kinds provide the same info. But it
will be more useful for identifying specific matrices such as the
upcoming NVLink one.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

API: add hwloc_distances_get_name() to retrieve the name of a distances
structure.
We don't add it to the public distances structure because it would
break the ABI.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

API: add hwloc_distances_get_by_name()
Useful for retrieving specific matrices such as "NUMALatency" or
"NVLink".
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

xml/export/distances: abstract-out the function that exports a single
distance structure
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

xml/import/distances2: add a missing error message
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

xml/import/distances2: fix some error messages
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>