Update SOMAXCONN value from Linux 5.4.

Linux 5.4 changes the SOMAXCONN value from 128 to 4096 (this isn't in
a uapi header; various constants related to the kernel/userspace
interface, including this one, are in the non-uapi linux/socket.h
header).

This patch increases the value in glibc.  As I understand it, it is
safe to use a higher value even with older kernels (the kernel will
simply adjust the value passed to listen to be no more than the value
supported in the kernel), and SOMAXCONN is actually only a default for
a sysctl value in the kernel that can be changed at runtime.  So I
think updating the value in glibc is a reasonable and safe thing to
do.

Tested for x86_64.