Cygwin: fhandler_pipe: fix permission problem

Cygwin: fhandler_pipe: fix permission problem

The read handles of pipes created by CreateNamedPipe don't have
FILE_WRITE_ATTRIBUTES access unless the pipe is created with
PIPE_ACCESS_DUPLEX.  This causes set_pipe_non_blocking to fail on such
handles.  To fix this, add a helper function nt_create, which uses
NtCreateNamedPipeFile instead of CreateNamedPipe and gives us more
flexibility in setting access rights.

Use this helper function in fhandler_pipe::create (fhandler_pipe *[2],
unsigned, int), which is the version of fhandler_pipe::create used by
the pipe and pipe2 system calls.

For convenience, also add a static member function
fhandler_pipe::npfs_handle similar to those used by fhandler_fifo and
fhandler_socket_unix.