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.