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.

tests/bison.in

@@ -34,6 +34,11 @@ if test -t 2; then
     shift
 fi
 
+# We redirect stderr, which breaks the computation of the terminal
+# screen width.  So export COLUMNS to Bison, hoping for the shell to
+# have defined it.
+: ${COLUMNS=132}
+export COLUMNS
 $PREBISON "$abs_top_builddir/src/bison" ${1+"$@"} 2>"$stderr"
 status=$?