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.

bootstrap.conf

@@ -45,6 +45,8 @@ gnulib_modules='
   unistd unistd-safer unlink unlocked-io
   update-copyright unsetenv verify
   warnings
+  winsz-ioctl
+  winsz-termios
   xalloc
   xalloc-die
   xconcat-filename