wg-quick and iptables race condition bug

[Ali Intika <intikaa@gmail.com>]

Hi,

First thanks for this amazing piece of code :)

The bug: iptables rules can not be applied in parallel, thus iptables
have the "-w" option (--wait, maximum wait to acquire xtables lock
before giving up) to circumvent this; the "-w" parameter is not used
in wireguard-tools which lead to a racing condition on some systems.

When using wg-quick on a system that do handle often the iptables
rules, especially if we are using wg-quick for a long manipulation (eg
turn up/down multiple interfaces), the command just fail if an other
iptables manipulation is going on, on the system. replacing "iptables"
with "iptables -w 10" on the Wireguard's script/c app (for android)
will solve the issue.

eg where the issue occur
- server handling iptables rules dynamically
- server/client where iptables is often manipulated

Personally I am mainly having this issue on android where the script
is a binary and not as easily editable as an sh file.

Thanks for your consideration
:)