diagnostics: keep the fixits

Introduce proper support for fixits, instead of just printing them on
demand.

* bootstrap.conf: We need gnulib's xlists.
* src/fixits.h, src/fixits.c: New.
* src/complain.c (deprecated_directive): Use fixits_register.
* src/main.c (main): Use fixits_free.

src/main.c

@@ -35,6 +35,7 @@
 #include "conflicts.h"
 #include "derives.h"
 #include "files.h"
+#include "fixits.h"
 #include "getargs.h"
 #include "gram.h"
 #include "lalr.h"
@@ -205,6 +206,7 @@ main (int argc, char *argv[])
   conflicts_free ();
   grammar_free ();
   output_file_names_free ();
+  fixits_free ();
 
   /* The scanner memory cannot be released right after parsing, as it
      contains things such as user actions, prologue, epilogue etc.  */