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.

m4/.gitignore

@@ -202,3 +202,5 @@
 /xalloc.m4
 /xsize.m4
 /xstrndup.m4
+/jm-winsz1.m4
+/jm-winsz2.m4