Add comments about the IGRP/EIGRP protocol number and #define mess.

Add comments about the IGRP/EIGRP protocol number and #define mess.

The current IANA protocol number assignments page lists 9 as "any
private interior gateway (used by Cisco for their IGRP)" and 88 as
"EIGRP" from Cisco.

Recent FreeBSD and DragonFly BSD <netinet/in.h> headers define
IP_PROTO_IGRP as 9 and IP_PROTO_IGRP as 88.  We define IP_PROTO_PIGP
("private interior gateway protocol") as 9 and IP_PROTO_EIGRP as 88;
those names* better match what the current protocol number assignments
say.