diagnostics: get the screen width from the terminal

* bootstrap.conf: We need winsz-ioctl and winsz-termios.
* src/location.c (columns): Use winsize to get the number of
columns.
Code taken from the GNU Coreutils.
* src/location.h, src/location.c (caret_init): New.
* src/complain.c (complain_init): Call it.
* tests/bison.in: Export COLUMNS so that users of tests/bison can
enjoy proper line truncation.

src/location.h

@@ -114,6 +114,9 @@ void location_compute (location *loc,
    Warning: uses quotearg's slot 3. */
 unsigned location_print (location loc, FILE *out);
 
+/* Prepare the use of location_caret.  */
+void caret_init (void);
+
 /* Free any allocated resources and close any open file handles that are
    left-over by the usage of location_caret.  */
 void caret_free (void);