ddclient: T5612: Enable TTL support for web-service based protocols

Enable TTL support for web-service based protocols in addition to
RFC2136 based (nsupdate) protocol.

Since TTL is not supported by all protocols, and thus cannot have a
configuration default, the existing XML snippet include/dns/time-to-live.xml.i
does not have common <defaultValue>300</defaultValue> anymore and is
instead added explicitly whenever necessary.