tools: Update sysctl to its new name

The kern.ipc.somaxconn sysctl has been replaced with
kern.ipc.soacceptqueue since FreeBSD 10.0.

Reviewed by:	emaste
MFC after:	1 hour
Differential Revision:	https://reviews.freebsd.org/D34598

tools/tools/netrate/tcpp/README

@@ -84,7 +84,7 @@ In my testing, I use loader.conf entries of:
 
 kern.ipc.maxsockets=1000000
 net.inet.tcp.maxtcptw=3000000
-kern.ipc.somaxconn=49152
+kern.ipc.soacceptqueue=49152
 kern.ipc.nmbjumbo16=262144
 kern.ipc.nmbjumbo9=262144
 kern.ipc.nmbjumbop=262144

tools/tools/sysdoc/tunables.mdoc

@@ -806,7 +806,7 @@ kern.ipc.nsfbufs
 kern.ipc.numopensockets
 
 ---
-kern.ipc.somaxconn
+kern.ipc.soacceptqueue
 int
 
 The maximum pending socket connection queue size.