dhcpv6: assign all viable DHCPv6 addresses by default (FS#402, FS#524)

dhcpv6: assign all viable DHCPv6 addresses by default (FS#402, FS#524)

Some client implementations expect DHCPv6 addresses to be assigned for
every prefix present in a RA as reported in FS#524; if not done the
client implementations spam the DHCPv6 server with SOLICIT messages
trying to get an DHCPv6 address for every prefix.
When ra_managed is set to 1 and an ULA prefix is configured; the DHCPv6
server will only assign an ULA DHCPv6 addresses which possibly leads to
DNS issues.

Fix these problems by providing dhcpv6_assignall config option which
assigns all viable DHCPv6 addresses to a client by default.
This is a trade off as it means client implementations will be assigned
more IPv6 addresses and lose the ability to quickly renumber clients
which don't support reconfigure accept in case of wan prefix change

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