pipe2: Remove dependency on 'nonblocking' module.

pipe2: Remove dependency on 'nonblocking' module.

* lib/pipe2.c: Include verify.h. Include nonblocking.h only if
O_NONBLOCK is defined by gnulib.
(pipe2) [WIN32]: If O_NONBLOCK is not defined by gnulib, verify that it
is zero.
* modules/pipe2 (Depends-on): Add verify. Remove nonblocking.
* tests/test-pipe2.c: Include nonblocking.h only if O_NONBLOCK is
defined by gnulib.
(get_nonblocking_flag): New function.
(main): Test O_NONBLOCK flag only if it is nonzero.
(cherry picked from commit 64e338384bc99dfb49c4f46648b3fa0c50e8491d)