wireguard: restart on failure

As a oneshot service, if the startup failed it would never be attempted again.
This is problematic when peer's addresses require DNS. DNS may not be reliably available at
the time wireguard starts. Converting this to a simple service with Restart
and RestartAfter directives allows the service to be reattempted, but at
the cost of losing the oneshot semantics.

Signed-off-by: Maximilian Bosch <maximilian@mbosch.me>