ra: align RA update interval with RFC4861 (FS#964)

ra: align RA update interval with RFC4861 (FS#964)

RFC4861 paragraph 6.2.1 states the minimum allowed time between sending
unsolicited multicast Router Advertisements is 3 seconds.
Align the odhcp6c implementation to this requirement as before Router
Advertisement updates were accepted each 30 seconds.
This lead to IPv6 connectivity issues when a network sends Router
Advertisements with an interval smaller than 30 seconds as reported
in FS#964.
While at it rework the RA update logic making it clear the -m option
is only related to RA updates.

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