Inet{4,6}AddressMatcher: netmask parser was way too permissive

It accepted invalid netmask like 192.168.1.0/100 (interpreting it as
192.0.0.0/4).

Discovered-by: ZeroFlag@B1g7PyGI4MI4ZNpEg8SleU58Rfs.frost
*allowedHosts config: throw correct exception on format errors

Note: IllegalArgumentException is also NumberFormatException superclass