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/basic/log.h

@@ -335,3 +335,5 @@ int log_syntax_internal(
                                             "String is not UTF-8 clean, ignoring assignment: %s", strna(_p)); \
                 }                                                       \
         })
+
+#define DEBUG_LOGGING _unlikely_(log_get_max_level() >= LOG_DEBUG)