warnings: sort the messages by location

Sort the warnings by location (file, line number, column). They are built
using an obstack then stored in a list, and finally sorted just before
being printed, at the end.

* src/complain.c, src/complain.h: New warning struct, obstack, and
implementation
* src/location.c, location.h (location_obstack_caret,
location_obstack_print): New
* src/main.c: Print the warnings
* src/muscle-tab.c, src/reader.c, src/scan-code.l, src/symtab.c: Adapt for
multi-part warnings
* tests/actions.at, tests/conflicts.at, tests/existing.at, tests/input.at,
* tests/reduce.at, tests/regression.at, tests/skeleton.at: Update testsuite

src/main.c

@@ -203,7 +203,6 @@ main (int argc, char *argv[])
      contains things such as user actions, prologue, epilogue etc.  */
   gram_scanner_free ();
   muscle_free ();
-  uniqstrs_free ();
   code_scanner_free ();
   skel_scanner_free ();
   quotearg_free ();
@@ -218,6 +217,9 @@ main (int argc, char *argv[])
   timevar_stop (TV_TOTAL);
   timevar_print (stderr);
 
+  print_warnings (stderr);
+
+  uniqstrs_free ();
   cleanup_caret ();
 
   return complaint_status ? EXIT_FAILURE : EXIT_SUCCESS;