fix: FTL_FNREDIR: don't depend on /dev/tty
fix: FTL_FNREDIR: don't depend on /dev/tty

bin/modernish:
- Relying on /dev/tty to be on a terminal turns out not to be a
  good idea because /dev/tty is not always usable, e.g. when a
  script is started from an editor such as BBEdit on the Mac. There
  is no tty associated with standard input in such a situation. So
  rewrite the FTL_FNREDIR to use /dev/null instead, and test if the
  file descriptor is opened by the redirection attached to the
  function definition. (This also eliminates another use of '['.)

modernish/modernish