netlink: allow path weight manipulations for single-path routes.

Add support for the scenario when user adds/deletes paths for a single
prefix one-by-one, all with different weights.
This change adds a new FreeBSD-specific RTA attribute, NL_RTA_WEIGHT.
When dumping non-multipath routes, this attribute is added if the
route weight is not RT_DEFAULT_WEIGHT.
When adding a new route, this attribute is parsed as a relative path
weight.

MFC after: 2 weeks

(cherry picked from commit 3ebccb20d56455f4bef1366f942680d1b60828f6)