Add wildcard support to rpcallowip

This patch allows you to use `*` with `-rpcallowip`

Internally , it converts a filter with wildcards to the internal ip address + netmask form. i.e. `192.168.*` becomes `192.168.0.0/16`

That means that this doesn't allow all uses of wildcards, but it's good enough 90% of the time.

It also improves the error message seen when parsing the subnet fails.

See also #4894.