fff-ra: use dhcp leasetime for preferred and valid lifetime

When advertising network prefixes gathered from the interface, odhcpd
sets the preferred and valid lifetime of those prefixes in the router
advertisement to the values set for those addresses on the interface.

When prefixes are configured statically (as done in our firmware), this
means that odhcp announces these prefixes for SLAAC with infinite
preferred and valid lifetimes.

While this does not seem like a problem at first, it hurts significantly
when configuration errors are made or cables are plugged into the wrong
ports, because those addresses never vanish from devices anymore, as long
as they are powered up. Also, it makes it impossible to change prefixes
without gracefully shutting down the RA server, so it can announce zero
lifetimes for previously announced prefixes.

Sadly, odhcp does not have an option to configure these lifetimes
explicitly, but it is possible to limit these lifetimes to the lease
time configured and used for the DHCP functionality of odhcpd.
Enable the appropriate 'ra_useleasetime' option to reduce impact of the
before mentioned problems.

Fixes: #142

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Christian Dresel <freifunk@dresel.systems>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>

src/packages/fff/fff-ra/files/etc/uci-defaults/90-fff-ra

@@ -4,6 +4,7 @@ uci batch <<EOF
 	set dhcp.client.ra='server'
 	set dhcp.client.ra_default='2'
 	set dhcp.client.ra_management='0'
+	set dhcp.client.ra_useleasetime='1'
 EOF
 
 exit 0