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.cpp

@@ -0,0 +1,58 @@
+#include "SdDaemon.hpp"
+
+#include <memory>
+
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+
+namespace Systemd {
+    int SdBooted(void) {
+        if (!faccessat(AT_FDCWD, "/run/systemd/system/", F_OK, AT_SYMLINK_NOFOLLOW))
+            return true;
+
+        if (errno == ENOENT)
+            return false;
+
+        return -errno;
+    }
+
+    int SdNotify(int unsetEnvironment, const char* state) {
+        int fd = socket(AF_UNIX, SOCK_DGRAM, 0);
+        if (fd == -1)
+            return -errno;
+
+        constexpr char envVar[] = "NOTIFY_SOCKET";
+
+        auto           cleanup = [unsetEnvironment, envVar](int* fd) {
+            if (unsetEnvironment)
+                unsetenv(envVar);
+            close(*fd);
+        };
+        std::unique_ptr<int, decltype(cleanup)> fdCleaup(&fd, cleanup);
+
+        const char*                             addr = getenv(envVar);
+        if (!addr)
+            return 0;
+
+        // address length must be at most this; see man 7 unix
+        size_t             addrLen = strnlen(addr, 107);
+
+        struct sockaddr_un unixAddr;
+        unixAddr.sun_family = AF_UNIX;
+        strncpy(unixAddr.sun_path, addr, addrLen);
+        if (unixAddr.sun_path[0] == '@')
+            unixAddr.sun_path[0] = '\0';
+
+        if (!connect(fd, (const sockaddr*)&unixAddr, sizeof(struct sockaddr_un)))
+            return 1;
+
+        // arbitrary value which seems to be enough for s-d messages
+        size_t stateLen = strnlen(state, 128);
+        if (write(fd, state, stateLen) >= 0)
+            return 1;
+
+        return -errno;
+    }
+}