wireguard: bump to 20180519

wireguard: bump to 20180519

* chacha20poly1305: add mips32 implementation

"The OpenWRT Commit" - this significantly speeds up performance on cheap
plastic MIPS routers, and presumably the remaining MIPS32r2 super computers
out there.

* timers: reinitialize state on init
* timers: round up instead of down in slack_time
* timers: remove slack_time
* timers: clear send_keepalive timer on sending handshake response
* timers: no need to clear keepalive in persistent keepalive

Andrew He and I have helped simplify the timers and remove some old warts,
making the whole system a bit easier to analyze.

* tools: fix errno propagation and messages

Error messages are now more coherent.

* device: remove allowedips before individual peers

This avoids an O(n^2) traversal in favor of an O(n) one. Before systems with
many peers would grind when deleting the interface.

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