check_fping: IPv6 support

Add command line options -4 and -6 (--use-ipv4 and
--use-ipv6) to check_fping. IPv4 is used by default,
maintaining backwards compatibility with older
check_fping versions. IPv6 support requires the
fping6 program.

The implementation is really just an imitation of
check_ping IPv6 support: Plugin command line option
-4/-6 designates whether to run fping/fping6.

Please note that especially the changes to
configure.in might be all wrong - I don't know
what's the proper autoconf way, patching
configure.in just seemed to work for me.