Set KillMode=mixed

Any child processes we fork are not long running, so they don't
need their own individual SIGTERM. Doing things this way ensures
that when we suppress SIGTERM for our process that's sufficient
to ensure the whole unit runs uninterrupted.

src/cli/bootupctl.rs

@@ -13,6 +13,10 @@ static SYSTEMD_ARGS_BOOTUPD: &[&str] = &[
     "PrivateNetwork=yes",
     "--property",
     "ProtectHome=yes",
+    // While only our main process during update catches SIGTERM, we don't
+    // want systemd to send it to other processes.
+    "--property",
+    "KillMode=mixed",
     "--property",
     "MountFlags=slave",
     "--pipe",