diagnostics: update the grammar file

Let's use the fixits to actually update the grammar files.

* src/getargs.h, src/getargs.c (update_flag): New.
* src/fixits.h, src/fixits.c (fixits_run): New.
* src/main.c (main): Invoke fixits_run when --update is passed.
* tests/input.at (Deprecated directives): Check --update.

bootstrap.conf

@@ -38,6 +38,7 @@ gnulib_modules='
   readme-release
   realloc-posix
   relocatable-prog relocatable-script
+  rename
   spawn-pipe stdbool stpcpy strdup-posix strerror strverscmp
   timevar
   unistd unistd-safer unlink unlocked-io