wireguard: bump to 0.0.20191205

wireguard: bump to 0.0.20191205

* wg-quick: linux: suppress error when finding unused table

This fixes a spurious warning messages seen with recent versions of iproute2
and kernels.

* wg-quick: linux: ensure postdown hooks execute
* wg-quick: linux: have remove_iptables return true
* wg-quick: linux: iptables-* -w is not widely supported

Adding in iptables had some hiccups. For the record, I'm very unhappy about
having to put any firewalling code into wg-quick(8). We'll of course need to
support nftables too at some point if this continues. I'm investigating with
upstream the possibility of adding a sysctl to patch the issue that iptables
is handling now, so hopefully at somepoint down the line we'll be able to shed
this dependency once again.

* send: use kfree_skb_list
* device: prepare skb_list_walk_safe for upstreaming
* send: avoid touching skb->{next,prev} directly

Suggestions from LKML.

* ipc: make sure userspace communication frees wgdevice

Free things properly on error paths.

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