rpc: Use netmasks instead of wildcards for IP address matching … 

As discussed on IRC.

`-rpcallowip` currently has a wacky wildcard-based format. After this commit it will accept the more standard format, for example:

- Ranges with netmask `127.0.0.0/255.255.255.0`, `::/0`
- Ranges with cidr `12.3.4.5/24`, `12:34:56:78:9a:bc:de:00/112`
- Loose IPs `::1`, `127.0.0.1`

Trying to use the old *?-based format will result in an error message at launch. Most people will likely be specifying single IPs, so won't notice this change at all.