pipe2-safer: new module

pipe2-safer: new module

pipe2 deserves a *_safer variant.  It also makes the code in
pipe.c look simpler.

* modules/pipe2-safer: New file.
* lib/unistd-safer.h (pipe2_safer): New prototype.
* lib/unistd--.h (pipe2): New wrapper.
* lib/pipe-safer.c (pipe2_safer): New function.
* modules/pipe (Depends-on): Add pipe2-safer.
* lib/pipe.c (create_pipe) [WIN32]: Let pipe2_safer do the work.

Signed-off-by: Eric Blake <ebb9@byu.net>