pipe-posix: Make it work in C++ mode.

pipe-posix: Make it work in C++ mode.

* lib/unistd.in.h: Don't include <io.h>, <fcntl.h> for pipe.
(pipe): Use common idiom, not a macro definition.
* lib/pipe.c: New file.
* m4/pipe.m4: New file.
* modules/pipe-posix (Description): Enhance.
(Files): Add lib/pipe.c, m4/pipe.m4.
(configure.ac): Invoke gl_FUNC_PIPE.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize HAVE_PIPE.
* modules/unistd (Makefile.am): Substitute HAVE_PIPE.
* tests/test-unistd-c++.cc: Check the signature of pipe.