network: honor M or O flag in RA even if IPv6AcceptRA.DHCPv6Cleint=always

Follow-up for ac24e418.

The original motivation of the commit and RFE #15339 is to start dhcpv6
client in managed mode when neither M nor O flag is set in the RA.
But, previously, if the setting is set to "always", then the DHCPv6
client is always started in managed mode even if O flag is set in the
RA. Such the behavior breaks RFC 7084.