router: support ra_mininterval and ra_lifetime uci parameters (FS#397)

Add support for uci parameters ra_mininterval and ra_lifetime as described
in RFC4861 paragraph 6.2.1.

Variable ra_mininterval allows to configure the minimum interval time
between unsolicited router advertisement messages; default value is
200 seconds. The minimum allowed value is 4 seconds while the maximum
value is limited to 0.75 of the maximum interval time.

The calculation of the maximum interval time between unsolicited
router advertisement messages has been reworked. The default value is 600
seconds as specified in RFC4861; if the maximum interval time exceeds
0.33 * the minimal valid lifetime of all IPv6 prefixes it will be limited
to 0.33 * the minimal valid lifetime of all IPv6 prefixes

Variable ra_lifetime allows to configure the Router Lifetime field
in the router advertisement messages; the value is either 0 or a value
between the maximum interval time and 9000 seconds. If the router lifetime
is smaller than the RA maximum interval it will be set equal to the RA
maximum interval time.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>