New module 'isatty'.

New module 'isatty'.

* lib/unistd.in.h (isatty): New declaration.
* lib/isatty.c: New file, based on an idea of
Bastien Roucariès <roucaries.bastien@gmail.com>.
* m4/isatty.m4: New file.
* m4/unistd_h.m4 (gl_UNISTD_H): Test whether isatty is declared.
(gl_UNISTD_H_DEFAULTS): Initialize GNULIB_ISATTY, REPLACE_ISATTY.
* modules/unistd (Makefile.am): Substitute GNULIB_ISATTY,
REPLACE_ISATTY.
* modules/isatty: New file.
* doc/posix-functions/isatty.texi: Mention the new module.
Suggested by Paolo Bonzini.