isatty - determine if a file unit is a terminal (teletypewriter)

[ivan-pi]

Suggested by @14NGiestas in fortran-lang/stdlib#193 (comment)

Many compilers already have their own extensions for this:

• GNU isatty
• Intel PXFISATTY
• PGI isatty
• Oracle isatty

Corresponding routines to query the terminal name:

• GNU ttynam
• Intel PXFTTYNAM
• PGI ttynam
• Oracle ttynam

The Intel run-time library actually provides these routines in two places, the IFPOSIX Posix-compliant library and the portabiliy library module IFPORT (under the names isatty and ttynam).

Linux man pages:

• isatty
• ttyname

Windows:

• _isatty
• GetConsoleTitle (apparently Windows is moving away from their Console API and the usage of virtual terminal sequences is preferred for new development for portability reasons)

Other possible enhancements could include functions to query the size of the terminal (an example can be found in @zbeekman's zstdlib project).