WIP dns/dyndns: tidy up code and add new service type Custom (v4/v6) to allow updating A and AAAA in one URL

[Starkstromkonsument]

• I rewrote parts of the scripts to be able to use IPv4 and IPv6 inside the same service type.
• Fixes a bug with "Force IPv4 resolving" for IPv6 service types. The switch only applied to CURL but not to retrieving the IP for the sending WAN interface. This ended up in CURL trying to send the update to an IPv4 address from an IPv6 Interface.
• The logging is tidied up
• The number of calls to _checkIP is reduced from three to one per update. This reduces the load at http://checkip.dyndns.org
• Refers to: https://forum.opnsense.org/index.php?topic=16948.msg89547
• closes dns/dyndns custom service types: include current AAAA or A record in update URL #1611
• closes dyndns result matching #555
• likely affects: Add IWNX to DDNS list #1636

[Starkstromkonsument]

I did not find any function to convert a bool to its string representation. If I missed something, please give me a hint and I'll be happy to use the existing function and remove my new one. THX

[AdSchellevis]

@Starkstromkonsument question is if it really matters if it's a bool, usually the only question is if it would render to "true", in which case I would use something like this !empty($var) ? "true" : "false" (or simpler, only log 0/1 using the same type of construct (int)!empty($var))

[Starkstromkonsument]

I guess keeping this PR open any longer does not make much sense. If I get this all right, developing a new plugin for ddclient would be the proper way to fix this. IPv6 support seems to be a current task for ddclient 4.0