wireguard service: use scripts instead of ExecStarts/Stops

This is more in line with what other services do; also looks cleaner.
It changes configuration entries for pre-and post-hooks type to lines from
lists of strings which are more logical for them; coersion is provided for
backwards compatibility.

Finally, add several steps to improve robustness:

1. Load kernel module on start if not loaded;
2. Don't remove wireguard interface on start; it is removed on service stop. If
   it's not something is wrong.