/kline parses strange cidr masks as their upper limits instead of rejecting

[AbstractBeliefs]

See #111 and its fix, #112 .

We should probably reject these klines rather than the current safe-but-incorrect behaviour.

[AbstractBeliefs]

As mentioned in #112, the main issue right now is that the parse_netmask() function used in src/hostmask.c returns only varying kinds of success, and has no way to indicate failure. Some work will need to go into updating the calling code of these functions to accept a failure value and do the right thing.

[ilbelkyr]

Also, per #112 (comment),

There appears to be an integer underflow with large negative numbers (n-2^32).
Example: @1.2.3.4/-4294967296 (n=0) matches all IPv4 on testnet.