Use a single timer for all ND6 entries.

Use a single timer for all ND6 entries.

This prevents a use-after-free reported by Hrvoje Popovski where the
timeout function was already sleeping on the NET_LOCK() when ifconfig(8)
removed the enry from the table.

By iterating on a global list in the timeout routine we ensure that the
items are still valid when we process them.  This also reduce differences
with ARP.

ok bluhm@, visa@