Escape unprintable characters when outputting filenames to a terminal.

Escape unprintable characters when outputting filenames to a terminal.

Filenames are generally untrusted, and can contain any kind of cruft.
In particular, there have been terminals (hopefully not in wide use anymore!)
that will do insanity like running specific commands when seeing a
specific escape sequence. More prosaically, embedded newlines can
make for confusing output.

Thus, escape any nonprintable characters in a shell-parseable way,
much the same way GNU ls does these days. Also escape quotes, backslashes
and the likes to make sure nothing unescaped looks like it's escaped.
This doesn't mean it's safe to take whatever and parse it uncritically
(we don't escape $, for instance), but it's generally good enough.

Escaping is disabled when doing zero-terminated output, or when printing
to a pipe or file.