Add stub for new plugin check_ping_multiaddr.

This adds the beginning of a new plugin, check_ping_multiaddr, which
is intended to be used to check that a host respond is alive and
responds to ping on all of its addresses, both IPv4 and IPv6, in a
single call.  What's added here is stub only implementing rudimentary
argument parsing, to give an idea of the user interface.

The intent is to use the fping(1) command for the actual pinging,
since it is intended to be used by other programs (and thus has
output reasonably easy to parse), and can check multiple addresses
in parallel.  (However, you need to use fping for IPv4 and fping6
for IPv6, so two invocations are needed.)

There are of course already many Nagios plugins for checking that a
host is alive out there, including the common 'check_ping' plugin from
the official Nagios Plugins project, and several others at the Nagios
Exchange, but none (that I have found) that can check both IPv4 and
IPv6 addresses in a single call.

Some alternatives examined and rejected:

 - check_mping  Only supports IPv4, and checks the hosts in serial.

 - check_icmp   Does check addresses in parallel, but only supports
                IPv4, and has uncertain copyright status (it is
                basically a hacked version of fping).  It also needs
                to be installed setuid root.

 - check_fping  Same as check_icmp.