BUG/MINOR: fcgi-app: prevent 'use-fcgi-app' in default section

Despite the doc saying that 'use-fcgi-app' keyword may only be used in backend
or listen section, we forgot to prevent its usage in default section.

This is wrong because fcgi relies on a filter, and filters cannot be defined in
a default section.

Making sure such usage reports an error to the user and complies with the doc.

This could be backported up to 2.2.
diff --git a/src/fcgi-app.c b/src/fcgi-app.c
index c9405ec..1ece72b 100644
--- a/src/fcgi-app.c
+++ b/src/fcgi-app.c
@@ -589,7 +589,7 @@
 	struct fcgi_flt_conf *fcgi_conf = NULL;
 	int retval = 0;
 
-	if (!(curpx->cap & PR_CAP_BE)) {
+	if ((curpx->cap & PR_CAP_DEF) || !(curpx->cap & PR_CAP_BE)) {
 		memprintf(err, "'%s' only available in backend or listen section", args[0]);
 		retval = -1;
 		goto end;