tree-wide: add DEBUG_LOGGING macro that checks whether debug logging is on (#7645)

This makes things a bit easier to read I think, and also makes sure we
always use the _unlikely_ wrapper around it, which so far we used
sometimes and other times we didn't. Let's clean that up.

src/core/execute.c

@@ -3418,7 +3418,7 @@ static int exec_child(
                 return log_oom();
         }
 
-        if (_unlikely_(log_get_max_level() >= LOG_DEBUG)) {
+        if (DEBUG_LOGGING) {
                 _cleanup_free_ char *line;
 
                 line = exec_command_line(final_argv);