isatty: Support for MSVC 9.

isatty: Support for MSVC 9.

* doc/posix-functions/isatty.texi: Mention the MSVC problem.
* lib/isatty.c: Include <errno.h>, msvc-inval.h.
(_isatty_nothrow): New function.
(isatty): Use it instead of _isatty.
(IsConsoleHandle): Add comment, from Paolo Bonzini.
* lib/poll.c (IsConsoleHandle): Likewise.
* lib/select.c (IsConsoleHandle): Likewise.
* m4/isatty.m4 (gl_FUNC_ISATTY): Fix comment. Reported by Eli Zaretskii.
(gl_PREREQ_ISATTY): New macro.
* modules/isatty (Depends-on): Add msvc-inval.
(configure.ac): Invoke gl_PREREQ_ISATTY.