Implement address lookup in check_ping_multiaddr.

This adds lookup of IP addresses from names of the hostnames/addresses
given on the command line, and errors out if the address cannot be
resolved.  By default, only the "first" address of each host will be
used; even if both --ipv4 and --ipv6 are specified, only one address,
family (the one the resolver library prefers) will be collected.
However, giving the -A/--all-addresses option will cause all addresses
(of the requested address families) for each hostname to be used.

We still don't actually do anything with the addresses yet, except
print them if the new -d/--debug option is used.