route: rtnl_route_build_msg() should not overwrite the route scope

rtnl_route_build_msg() should allow the user to set the route scope
explicitly to RT_SCOPE_NOWHERE.

This is useful for IPv4 routes, because when deleting a route,
the kernel requires the scope to match, unless the scope is set to
RT_SCOPE_NOWHERE. Thus by setting the scope to RT_SCOPE_NOWHERE,
the user can delete a route, even without knowing its scope.

rtnl_route_build_msg() should only try to guess the scope, if it was
not explicitly specified.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>