MINOR: filters: extend flt_dump_kws() to dump to stdout

When passing a NULL output buffer the function will now dump to stdout
with a more compact format that is more suitable for machine processing.

An entry was added to dump_registered_keyword() to call it when the
keyword class "flt" is requested.
diff --git a/src/filters.c b/src/filters.c
index 06ae788..4594098 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -130,6 +130,7 @@
 /*
  * Dumps all registered "filter" keywords to the <out> string pointer. The
  * unsupported keywords are only dumped if their supported form was not found.
+ * If <out> is NULL, the output is emitted using a more compact format on stdout.
  */
 void
 flt_dump_kws(char **out)
@@ -137,18 +138,20 @@
 	struct flt_kw_list *kwl;
 	int index;
 
-	if (!out)
-		return;
-
-	*out = NULL;
+	if (out)
+		*out = NULL;
 	list_for_each_entry(kwl, &flt_keywords.list, list) {
 		for (index = 0; kwl->kw[index].kw != NULL; index++) {
 			if (kwl->kw[index].parse ||
 			    flt_find_kw(kwl->kw[index].kw) == &kwl->kw[index]) {
-				memprintf(out, "%s[%4s] %s%s\n", *out ? *out : "",
-				          kwl->scope,
-				          kwl->kw[index].kw,
-				          kwl->kw[index].parse ? "" : " (not supported)");
+				if (out)
+					memprintf(out, "%s[%4s] %s%s\n", *out ? *out : "",
+					          kwl->scope,
+					          kwl->kw[index].kw,
+					          kwl->kw[index].parse ? "" : " (not supported)");
+				else
+					printf("%s [%s]\n",
+					       kwl->kw[index].kw, kwl->scope);
 			}
 		}
 	}
diff --git a/src/haproxy.c b/src/haproxy.c
index b5f368d..6d28c52 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1826,6 +1826,7 @@
 			printf("# List of supported keyword classes:\n");
 			printf("all: list all keywords\n");
 			printf("cfg: configuration keywords\n");
+			printf("flt: filter names\n");
 			continue;
 		}
 		else if (strcmp(kwd_dump, "all") == 0) {
@@ -1836,6 +1837,11 @@
 			printf("# List of registered configuration keywords:\n");
 			cfg_dump_registered_keywords();
 		}
+
+		if (all || strcmp(kwd_dump, "flt") == 0) {
+			printf("# List of registered filter names:\n");
+			flt_dump_kws(NULL);
+		}
 	}
 }