FTL_CASEBKSL: fatal bug with 'case' in Busybox 1.28.0
FTL_CASEBKSL: fatal bug with 'case' in Busybox 1.28.0

bin/modernish:
- Added FTL_CASEBKSL fatal bug test.

  The following outputs BUG in Busybox ash 1.28.0:
    case \\z in
    "\z" )  echo ok ;;
    * )     echo BUG ;;
    esac

  Apparently `case` has trouble matching the "\z" pattern due to
  the backslash within the double quotes. Under POSIX it is not
  needed to escape that backslash there, as the character
  following it is not one of $, `, ", \ or <newline>.

  This is a bug that doesn't just cause problems for modernish but
  also for scripts using it, and modernish can do nothing to
  implement a workaround that works for scripts and not just
  itself. So, unfortunately we must detect a fatal shell bug in the
  latest Busybox ash version. (Busybox ash 1.27.0 works fine and is
  supported.)

  Ref.:
  http://lists.busybox.net/pipermail/busybox/2018-February/086223.html

modernish/modernish