unix: sanity-check fds before closing

Ensure that close() system calls don't close stdio file descriptors
because that is almost never the intention.

This is also a partial workaround for a kernel bug that seems to affect
all Linux kernels when stdin is a pipe that gets closed: fd 0 keeps
signalling EPOLLHUP but a subsequent call to epoll_ctl(EPOLL_CTL_DEL)
fails with EBADF.  See joyent/node#6271 for details and a test case.
unix: remove uv__pipe_accept()

It's basically a less advanced version of uv__server_io().  Drop the
former in favor of the latter.