MINOR: filters: Print the list of existing filters during HA startup

This is done  in verbose/debug mode and when build options are reported.
diff --git a/src/filters.c b/src/filters.c
index 420ad80..a8efaa1 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -151,6 +151,21 @@
 }
 
 /*
+ * Lists the known filters on <out>
+ */
+void
+list_filters(FILE *out)
+{
+	char *filters, *p, *f;
+
+	fprintf(out, "Available filters :\n");
+	flt_dump_kws(&filters);
+	for (p = filters; (f = strtok_r(p,"\n",&p));)
+		fprintf(out, "\t%s\n", f);
+	free(filters);
+}
+
+/*
  * Parses the "filter" keyword. All keywords must be handled by filters
  * themselves
  */