Never send boot process stdio to application stdio.
Never send boot process stdio to application stdio.

We treat handle the boot process stdio separately from the application stdio
(which gets passed via flags), but we were still sending both to same place. As
a result, some logs that are written directly to os.Stderr by the boot process
were ending up in the application logs.

This CL starts sendind boot process stdio to the null device (since we don't
have any better options). The boot process is already configured to send all
logs (and panics) to the log file, so we won't miss anything important.

PiperOrigin-RevId: 217173020
Change-Id: I5ab980da037f34620e7861a3736ba09c18d73794

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor