INET4 data type

Under terms of MDEV-274 we added the INET6 data type, to store IPv6 addresses as 16-byte binary strings.

Let's also add a data type to store IPv4 addresses, as 4-byte binary strings.

The new data type name will be INET4.

Under terms of this task, INET4 and INET6 won't support mutual explicit or implicit CAST. That will be done separately when needed.

But INET4 should be implemented in the same directory with INET6: plugin/type_inet/, so later INET4 and INET6 "know" each other, to implement mutual cast.