IP address resolution using one external program

IP address resolution using one external program

It is now possible to resolve an IP address using an external program.
Only one external program can be configured but it may do anything
including attempting several strategies to resolve the IP address.

The module may be chained after the standard dns module to get the name of
a computer not registered with the DNS.

Executing an external program is exceedingly slow so it is best to try
the DNS first!