Motivation for this change

Previously each oneshot peer service only ran once and was not
restarted together with the interface unit. Because of this,
defined peers were missing after restarting their corresponding
interface unit.

The issue should be easily reproducible:

[root@nixos]# wg show wg0
interface: wg0
  public key: <pubkey>
  private key: (hidden)
  listening port: 12345

peer: <peer-pubkey>
  endpoint: <endpoint>
  allowed ips: <allowed-ips>
  latest handshake: 1 second ago
  transfer: 2.25 KiB received, 708 B sent

[root@nixos]# systemctl stop wireguard-wg0.service

[root@nixos]# systemctl start wireguard-wg0.service

[root@nixos]# wg show wg0
interface: wg0
  public key: <pubkey>
  private key: (hidden)
  listening port: 12345

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

/cc @grahamc @fpletz