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.