core: remove libsystemd dependency (#5660)

* remove libsystemd dependency

as per Lennart Poettering's advice:
https://github.com/systemd/systemd/issues/32028#issuecomment-2031366922

* fix naming for systemd helper functions

* rename vars according to code style

* Nix: update meson patch

---------

Co-authored-by: Mihai Fufezan <mihai@fufexan.net>

src/helpers/SdDaemon.hpp

@@ -0,0 +1,6 @@
+#pragma once
+
+namespace Systemd {
+    int SdBooted(void);
+    int SdNotify(int unset_environment, const char* state);
+}