delayed DNS lookup for IP addresses

[Carter Bullard]

Gentle people,
In the next round of clients, I have implemented a DNS lookup thread
that does name resolution for IPv4 addresses that is non-blocking.
This causes ra* programs to print dot notation addresses until the
reverse lookup thread finally gets the address resolved, and then
it starts printing FQDN.   We try once to resolve the address, so we
don't generate gazillion's of requests for the same address.  Because
of this, I took out the "setjmp(), alarm()" strategy we were using,
to limit the waiting time for an address to 5 seconds.

For programs like ratop() this is great, because the tool chugs away
asynchronously replacing the dot notation with the name
when it finally figures it out.  For programs like ra(), its hard to  
know
how useful it is, since in the case of reading files, you could have the
file read, before a single IP address is resolved.   I suspect this  
should
be a selectable option, or some programs should use it and some
should not.

Consider this a test of the feature, if it works well, I'll shift it  
to the IPv6
routines.   It is easy to revert back to the older way of doing things,
so we could make it a .rarc selectable mode of operation?

This is important for the real-time programs like ratop(), which is the
primary program I use for looking at argus data.

Suggestions. opinions, comments?

Carter