sys/netinet6/nd6.c: add ND6_IFF_ACCEPT_RTADV flag

[Jeffrey Hsu]

Hiroki Sato wrote:

 For a multi-homed IPv6 host with multiple NICs, that the host
 accepts all RAs can be a problem because they are not always
 trusted and can confuse the routing table and so on.  While
 the autoconfiguration of IPv6 using RAs itself can be performed
 independently on each interface, if the administrator cannot
 control which interface accepts RAs (currently "all" or "nothing" can
 be set via sysctl), he cannot connect the host to untrusted
 IPv6 network, for example.
Great, committed!  Thanks for your submission.

						Jeffrey