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.
Merge remote-tracking branch 'github/villemattila/check_fping_ipv6'

* github/villemattila/check_fping_ipv6:
  check_fping: IPv6 support

Conflicts:
	plugins/check_fping.c
check_fping: Use fping6(8) only if available

Let check_fping use IPv6 only if

- the operating system supports IPv6,
- the user didn't specify --without-ipv6, and
- the "configure" script found the fping6(8) binary.
NEWS: Mention the new check_fping features