This is sometimes useful after a mobility event, but it increases the
amount of control traffic when a route is unstable.  Let the periodic
update take care of mobility.

This never triggerred, since send_ihu is never called with no interface.

The source table is now sorted, just like the route table.  Source insertion
is still linear, and source expiry quadratic, but this hopefully doesn't
matter much.

This appears to get rid with ENOBUFS on recvmsg.  Kill me if I know why.

Two main modifications:
  - source-specific routes are putted in front of the RIB,
  - route_stream now allows traversing all routes, all installed routes,
    or source-specific installed routes.

WLAN-SI are running into the limit.  Cool.

- We keep the "export-table" option for compatibility reasons.

- It overrides the selection of source-specific tables.  Not sure if it
  is the right thing to do.

Thanks to Jernej Kos <jernej@kos.mx> for its review.

The previous patch to only write sysctls when needed failed to restore
values that was 0 before being set by babeld. This fixes that.
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>