Import the <netinet/in.h> from current FreeBSD. This allows to build
some current software (e.g. libressl).
Add legacy support like
* prototype for in_cksum(),
* IPPORT_USERRESERVED,
* deprecated IPCTL_RT* defines,
* ip_fw_chk_t and ip_fw_ctl_t,
* ip_nat_... (IP NAT hooks), and
* IP_NAT option for get/setsockopt()
to new <rtems/rtems_netinet_in.h>.