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/reader.c

@@ -129,7 +129,7 @@ record_merge_function_type (int merger, uniqstr type, location declaration_loc)
   if (merge_function->type != NULL && !UNIQSTR_EQ (merge_function->type, type))
     {
       unsigned indent = 0;
-      complain_indent (&declaration_loc, complaint, &indent,
+      start_complain_indent (&declaration_loc, complaint, &indent,
                        _("result type clash on merge function %s: "
                          "<%s> != <%s>"),
                        quote (merge_function->name), type,