Implement debuglog command

Add debuglog command that takes toggle|on|off. Add get_debug_logging()
to be able to toggle. Make t/187-commands-parser.t expect 'debuglog'.
Document the debuglog command in userguide.

src/log.c

@@ -180,6 +180,14 @@ void set_verbosity(bool _verbose) {
     verbose = _verbose;
 }
 
+/*
+ * Get debug logging.
+ *
+ */
+bool get_debug_logging(void) {
+    return debug_logging;
+}
+
 /*
  * Set debug logging.
  *