net: Add CSubNet class for subnet matching
rpc: Use netmasks instead of wildcards for IP address matching

`-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.
Remove unused function WildcardMatch

No longer necessary after implementing netmask-based matching.