Warn if SO_REUSEPORT cannot be set
On some kernels prior to Linux 3.9 (e.g. Oracle Linux v6.5), this kernel feature may not be available. Instead of failing hard, we just display a warning message that no-downtime upgrades will not work properly.
After we deprecate CentOS 6 (omnibus-gitlab#5163 (closed)), we can revert this.
Closes gitlab#219080 (closed)
Edited by GitLab Release Tools Bot