ndp: implement delayed anycast and proxy NA

[pouria]

Summary

the following items are implemented:

• proxy advertisement delay rule (RFC4861 7.2.8, last paragraph, SHOULD)
• anycast advertisement delay rule (RFC4861 7.2.7, SHOULD)

Test Plan

set an anycast address on host A

ifconfig vtnet0 inet6 3fff::1 anycast

ping it from host B

ping -c2 3fff::1

your first response must have a random delay between 0 and 1 seconds.
repeat this process by removing the ndp cache from host B and you must see a
different response time.

ndp -d 3fff::1
ping -c2 3fff::1

[pouria]

Update by rebase to main. follow update by parent revision.

[pouria]

@glebius this revision is an extention of GRAND which only affects anycast and proxy ipv6 addresses.
Could you please review it c-wise?