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/complain.c

@@ -308,6 +308,8 @@ complain_init_color (void)
 void
 complain_init (void)
 {
+  caret_init ();
+
   warnings warnings_default =
     Wconflicts_sr | Wconflicts_rr | Wdeprecated | Wother;