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/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);
+		}
 	}
 }