netstat: export weight for single-path routes.

• Parse and export newly-added NL_RTA_WEIGHT attribute, providing path weight for a non-multipath route. This fixes a number of tests in sys/net/routing which rely on this data.
• Remove handling of NL_RTA_KNH_ID in multipath routes, as it is not provided.
• Improve kernel/user nexthop index export. As a result, for multipath routes:
  • nhg-kidx attribute represents kernel nhg index (always provided)
  • nhg-uidx attribute represents user-provided nhg index (if set) for non-multipath routes:
  • nhop-kidx attribute represents kernel nhop index (always provided)
  • nhop-udx attribute represents user-provided nexthop index (if set)