purposes of quotearg

Hi,

I would like to document, in quotearg.h, the possible purposes of the
'quotearg' module.

Clearly, not all *_quoting_style styles fulfil all the possible purposes.
But once we have a list of possible purposes, we can make up the matrix
which style fulfils which purposes.

The purposes I can think of are the following:

  * Avoid ambiguous diagnostics. For instance, if 'E.T.' is the name of
    a network interface:
      program: cannot connect to E.T.
    is ambiguous, whereas
      program: cannot connect to 'E.T.'
    is less ambiguous.

  * Avoid emitting escape sequences that, by virtue of being ANSI
    escape sequences <https://en.wikipedia.org/wiki/ANSI_escape_code>,
    may cause trouble in the terminal, including (but not limited to)
      - displaying fake results,
      - crashing the terminal.
    For instance, create a file like this:
      $ touch `printf 'fooinv\x1b[3Dbar'`
    Then some diagnostics might make the user think that they are about
    'foobar'.

  * Avoid printing characters that are "unprintable" in the terminal's
    encoding. If that encoding is not UTF-8, they would typically leave
    no traces on screen, thus end up presenting misleading results.

Other purposes that I missed?

Bruno