A relay's Settings gain "Who may link": linkAllowFrom, the addresses and
ranges the relay takes this server's link from; empty is anyone. Shown
only where the server has the setting. A list that doesn't read is
refused before it is sent.
Under it, a warning when the list leaves out an address the relay last
saw a node link from (links[].relay.linkFrom): for every node, that it
locks this server out and --link-allow-any undoes it; for some, which
nodes couldn't link again. Addresses are matched as the relay matches
them, an IPv4-mapped IPv6 address as IPv4.