tools/libs/foreignmemory: Support err == NULL to map.

tools/libs/foreignmemory: Support err == NULL to map.

The existing xc_map_foreign_bulk-like interface encourages callers to
miss error checking for partial failure (by forgetting to scan the err
array).

Add support for passing err==NULL which behaves in a
xc_map_foreign_pages-like manner and returns a global error for any
failure.

While documenting this also clarify the overall behaviour and the
behaviour with err!=NULL.

With this the compat wrapper of xc_map_foreign_pages() can be
simplified.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>