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

(cherry picked from commit 84333aada464296c1e28126b5c91ae2fb3f79eef)