Fix interpretation of TTL as UINT32 with most significant bit unset

[clue]

TTL is actually a UINT32 with most significant bit unset for BC reasons as per https://tools.ietf.org/html/rfc2181#section-8. This will not affect "normal" operation as the most significant bit should not be set by compliant implementations and we will now interpret this accordingly.

Refs #81
Refs #114

[kelunik]

It's probably best to limit the TTL to a much lower value, such as one day, to mitigate the effect of a successful cache poisoning attack.

[clue]

@kelunik I think you're raising a valid point when it comes to the caching layer (see #81). However, this PR is concerned with the networking layer. It's my understanding that this layer should therefor implement the DNS protocol specs as suggested in this PR.

[kelunik]

@kelunik Indeed, the policy for that shouldn't live in the parser.