Send prompt to stdout even if stdin is not a tty or check TERM also on Windows

[jasagredo]

This issue arises from the investigation in mthom/scryer-prolog#2694. There is a comment at the end explaining my conclusions but I will re-word it here below.

I was having issues calling Scryer Prolog from emacs. Scryer Prolog uses rustyline for the repl, and it will print a prompt on the repl (?- ). The emacs package ediprolog expects that prompt for running.

On Linux, the emacs shell eshell is identified by rustyline as an unsupported terminal, as it sets TERM=dumb. There rustyline flushes the prompt to stdout.

On Windows, the emacs shell eshell also sets TERM=dumb but this is not checked (it is said that checking TERM on Windows is a no-op, not sure why) so it falls into the third case of readline_with which does not flush the prompt.

I think at least with MSYS2 and others it is becoming more common for Windows applications to use things like TERM=dumb so I guess the check for "unsupported terminals" could be run also on Windows?

[gwenn]

For is_unsupported, we need to check the behavior of other similar libraries but your proposal seems right.
For MSYS2, see #401.

[gwenn]

Both replxx and isocline seem to behave like you suggested: no distinction between unix and windows.

[gwenn]

Could you please give a try to PR #834 ?
Thanks.

[jasagredo]

Thanks @gwenn !