Medium: ldirectord: Enhance IPv6 support

• Added new directives: virtual6, real6, fallback6. These are used to specify IPv6 as addresses or hostnames. Now cannot specify an IPv6 address in a "virtual" and "real" line (This breaks backwards compatibility).
• Supported following checktypes and services. checktype: connect, external, external-perl, negotiate, off, on, checktimeoutN service: dns, nntp, none, simpletcp, sip
• firewall-mark + IPv6 works.

Signed-off-by: Simon Horman <horms@verge.net.au>