make --columns default to terminal width, as returned by ioctl()

If set, the COLUMNS environment variable will take precedence over terminal
width. However, please note that COLUMNS is usually *not* exported by shells to
child processes, so in most cases COLUMNS will be undefined for ledger---hence
the motivation for this change.

Terminal width is queried using ioctl() on stdin. For the sake of portability
the querying is done only on platform where ioctl() is detected as supported at
compile-time.