wireguard: bump to 0.0.20180708

wireguard: bump to 0.0.20180708

* device: print daddr not saddr in missing peer error
* receive: style

Debug messages now make sense again.

* wg-quick: android: support excluding applications

Android now supports excluding certain apps (uids) from the tunnel.

* selftest: ratelimiter: improve chance of success via retry
* qemu: bump default kernel version
* qemu: decide debug kernel based on KERNEL_VERSION

Some improvements to our testing infrastructure.

* receive: use NAPI on the receive path

This is a big change that should both improve preemption latency (by not
disabling it unconditionally) and vastly improve rx performance on most
systems by using NAPI. The main purpose of this snapshot is to test out this
technique.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>