sys/socket.h: Fix AF_MAX

[des]

Summary

AF_MAX was always intended to be one more than the greatest allocated
value. Jeff broke this in 2013. Unfortunately, a bunch of people then
decided to adapt to the mistake instead of fixing it.

Fixes: 863c7e45628d (" - Reserve a special AF for SDP. The one we were incorrectly using before was taken by another AF.")
MFC after: 3 days
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.

[bms]

This is directly in the line of fire of my GSoC student's MPLS work.

[kevans]

In D58597#1344902, @bms wrote:

This is directly in the line of fire of my GSoC student's MPLS work.

This should be pretty easy to rebase around, at least.

[bms]

In D58597#1345174, @kevans wrote:

In D58597#1344902, @bms wrote:

This is directly in the line of fire of my GSoC student's MPLS work.

This should be pretty easy to rebase around, at least.

Yeah, I don't doubt it, but I did get Redmond flashbacks.