Support non-blocking pipe I/O in write() on native Windows.

Support non-blocking pipe I/O in write() on native Windows.

* lib/unistd.in.h (write): Enable replacement also if
GNULIB_UNISTD_H_NONBLOCKING is 1.
* lib/write.c: Enable replacement also if GNULIB_NONBLOCKING.
(rpl_write): When failing to write on a non-blocking pipe, change
errno from ENOSPC to EAGAIN.
* lib/stdio.in.h (fprintf, fputc, fputs, fwrite, printf, putc,
putchar, puts, vfprintf, vprintf): Enable replacement also if
GNULIB_STDIO_H_NONBLOCKING is 1.
* lib/stdio-write.c: Enable replacements also if GNULIB_NONBLOCKING.
(CLEAR_ERRNO, HANDLE_ENOSPC): New macros.
(CLEAR_LastError, HANDLE_ERROR_NO_DATA): New macros, extracted from
CALL_WITH_SIGPIPE_EMULATION.
(CALL_WITH_SIGPIPE_EMULATION): Use them.
* m4/nonblocking.m4: New file.
* m4/write.m4 (gl_FUNC_WRITE): Enable REPLACE_WRITE also if required
for non-blocking I/O support.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize
GNULIB_UNISTD_H_NONBLOCKING.
* m4/stdio_h.m4 (gl_STDIO_H): Enable REPLACE_STDIO_WRITE_FUNCS also if
required for non-blocking I/O support.
(gl_STDIO_H_DEFAULTS): Initialize GNULIB_STDIO_H_NONBLOCKING.
* modules/nonblocking (Files): Add m4/nonblocking.m4,
lib/stdio-write.c, m4/asm-underscore.m4.
(Depends-on): Add stdio, unistd.
(configure.ac): Invoke gl_NONBLOCKING_IO. Define GNULIB_NONBLOCKING.
Set GNULIB_STDIO_H_NONBLOCKING, GNULIB_UNISTD_H_NONBLOCKING.
* modules/unistd (Makefile.am): Substitute GNULIB_UNISTD_H_NONBLOCKING.
* modules/stdio (Makefile.am): Substitute GNULIB_STDIO_H_NONBLOCKING.
* doc/posix-functions/fprintf.texi: Mention 'nonblocking' module and
problem with non-blocking pipes.
* doc/posix-functions/fputc.texi: Likewise.
* doc/posix-functions/fputs.texi: Likewise.
* doc/posix-functions/fwrite.texi: Likewise.
* doc/posix-functions/printf.texi: Likewise.
* doc/posix-functions/putc.texi: Likewise.
* doc/posix-functions/putchar.texi: Likewise.
* doc/posix-functions/puts.texi: Likewise.
* doc/posix-functions/vfprintf.texi: Likewise.
* doc/posix-functions/vprintf.texi: Likewise.
* doc/posix-functions/write.texi: Likewise.