The system call takes an int, but the kernel stores it in a uint16.
At least one Linux system sets /proc/sys/net/core/somaxconn
to 262144, which ends up being 0 in the uint16. Avoid being tricked.

FreeBSD sources also store the backlog in a uint16.
Assume the problem is systemic and fix it everywhere.

Fixes #5030.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7480046