unpack: warn for unofficial sufffix, bug #476738
Use F_GETFD/F_SETFD for FD_CLOEXEC.

This mixup may have caused bug #456296 (see commit
e43524dc88774f768441dcb386a534166a53f7fa).
_setup_pipes: os.set_inheritable() for Python 3.4
_setup_pipes: copy descriptor flags after dup2
FakeVartree: infinite _aux_get_wrapper recursion
_setup_pipes: fix for Python 3.1 to 3.3

This fixes a case where /dev/null stdin fails to inherit since commit
bd2128b7a750a68470f857162784e55b8ac39de2.