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.