util: move logind_running() to login-util.[ch]

src/basic/login-util.h

@@ -22,5 +22,10 @@
 #pragma once
 
 #include <stdbool.h>
+#include <unistd.h>
 
 bool session_id_valid(const char *id);
+
+static inline bool logind_running(void) {
+        return access("/run/systemd/seats/", F_OK) >= 0;
+}