Increase kernel buffers for UDP

Suggested sysctl.conf parameters for better handling of UDP packets:

net.core.rmem_max=26214400 // BDP - bandwidth delay product
net.core.rmem_default=26214400
net.core.wmem_max=26214400
net.core.wmem_default=26214400
net.core.netdev_max_backlog=2048 // proportional to -rcvwnd

useful pointers:

• https://medium.com/@CameronSparr/increase-os-udp-buffers-to-improve-performance-51d167bb1360
• https://github.com/xtaci/kcptun