Babel: Immediately update hello interval on interface reconfigure

An interface reconfiguration may change both the hello and update
intervals. An update interval change is immediately put into effect,
while a hello interval change is not. This also updates the hello
interval immediately (if the new interval is shorter than the old one),
and sends a hello to notify peers of the change.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>