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.

bootstrap.conf

@@ -17,7 +17,7 @@
 
 # gnulib modules used by this package.
 gnulib_modules='
-  argmatch assert
+  argmatch array-list assert
   bitsetv
   calloc-posix close closeout config-h c-strcase
   configmake
@@ -26,7 +26,9 @@ gnulib_modules='
   getopt-gnu
   gettext git-version-gen gitlog-to-changelog
   gpl-3.0 hash inttypes isnan javacomp-script
-  javaexec-script ldexpl malloc-gnu
+  javaexec-script
+  ldexpl
+  malloc-gnu
   mbswidth
   non-recursive-gnulib-prefix-hack
   obstack
@@ -44,6 +46,7 @@ gnulib_modules='
   xalloc
   xalloc-die
   xconcat-filename
+  xlist
   xmemdup0
   xstrndup