init: work around the mother of all race conditions

[lmb]

ebpf-go CI has been plagued by sporadic hangs, where tests simply time out while trying to write status information to stdout.

The bug manifests when issuing blocking writes to a virtio console while also polling it. The way we trigger the bug is quite involved:

• init opens tha port via os.OpenFile. This sets O_NONBLOCK on the fd, and registers the os.File with the poller.
• The port is passed to the child process via exec.Cmd.Stdout. This internally calls os.File.Fd(), which clears O_NONBLOCK but doesn't remove the file from the poller.
• The child process receives a blocking stdout. Writing to it will issue a blocking write to the virtio-console port, specifically port_fops_write() in virtio_console.c.
• port_fops_write() calls wait_port_writable(). This puts the calling thread to sleep if the virtqueue is full, by waiting on port->waitqueue.

We now enter the race window.

• The host processes the guest's write, frees up some space in the virtqueue and issues an interrupt to the guest.
• This interrupt races with a call to port_fops_poll() issued by the init process's Go runtime. That function invokes will_write_block(), which consumes all used buffers from the virtqueue.
• The interrupt handler vring_interrupt() checks whether the virtqueue has any unused buffers via more_used(). Since all buffers have just been consumed by port_fops_poll() the interrupt is dropped.

At this point we still have a writer stuck in port_fops_write() waiting for a wakeup that never comes.

The workaround for this issue is to close the stdio file in init, thereby removing it from the runtime poller.

Fixes: #29