add source line number preservation

For source-level debug. Add --line-directives option, defaulted to
statement. Disabled when used with -C. Explicit non-none value
overrides disabling when -C is present

sex-fmt-c.scm

@@ -845,8 +845,7 @@
 
   (define (c-label name)
     (lambda (st)
-      (let ((indent (make-space (max 0 (- (fmt-col st) 2)))))
-        ((cat fl indent name ":" fl) st))))
+      ((cat name ":" fl) st)))
 
   (define c-break
     (c-wrap-stmt (dsp "break")))