automatically shut down service after main function returns

[mazunki]

Builds and tests succesfully on top of #2363 due to tests.

This PR addresses a point in #2364 where the README advertises that the following program is a minimal valid program:

#include <os>
int main() {
  std::println("Hello");
}

Before, this program would leave the kernel hanging, without shutting down.

If Service::start() is defined, on the other hand, this responsibility remains on the service.