tree-wide: also settle on "initrd" instead of "initial RAM disk"

With this the concept is now called the same way everywhere except where
historical info is relevant or where the other names are API.

src/core/main.c

@@ -1858,7 +1858,7 @@ static int do_reexecute(
                 args[i++] = argv[j];
         assert(i <= args_size);
 
-        /* Re-enable any blocked signals, especially important if we switch from initial ramdisk to init=... */
+        /* Re-enable any blocked signals, especially important if we switch from initrd to init=... */
         (void) reset_all_signal_handlers();
         (void) reset_signal_mask();
         (void) rlimit_nofile_safe();
@@ -2052,7 +2052,7 @@ static void log_execution_mode(bool *ret_first_boot) {
 
                 if (in_initrd()) {
                         *ret_first_boot = false;
-                        log_info("Running in initial RAM disk.");
+                        log_info("Running in initrd.");
                 } else {
                         int r;
                         _cleanup_free_ char *id_text = NULL;