List: reslist cleanup

[Hal Murray]

This is just the tip of an iceberg:
  ntpsec | unpeer doesn't remove server from reslist (#315)
  Issue 315: https://gitlab.com/NTPsec/ntpsec/issues/315

Background:
  If you get a server or pool address via DNS lookup, it checks to see if 
replies from that address would get blocked by the current restrictions.  If 
yes, it pokes a hole in the restrictions to let them through.

Aside from unpeer leaving them behind, they will also get left if a pool 
server that is not responding gets dropped.

I think it takes another flag to mark a restriction as automatically added so 
we know we should delete it.

I think we need another flag to say "don't poke a hole through this slot."
 

-- 
These are my opinions.  I hate spam.