Fix route flags update during RTM_CHANGE.

Fix route flags update during RTM_CHANGE.
Nexthop lookup was not consireding rt_flags when doing
 structure comparison, which lead to an original nexthop
 selection when changing flags. Fix the case by adding
 rt_flags field into comparison and rearranging nhop_priv
 fields to allow for efficient matching.
Fix `route change X/Y flags` case - recent changes
 disallowed specifying RTF_GATEWAY flag without actual gateway.
 It turns out, route(8) fills in RTF_GATEWAY by default, unless
 -interface flag is specified. Fix regression by clearing
 RTF_GATEWAY flag instead of failing.
Fix route flag reporting in RTM_CHANGE messages by explicitly
 updating rtm_flags after operation competion.
Add IPv4/IPv6 tests for flag-only route changes.