Instead of exposing to consumers write streaming log data into a per-device
buffer that gets created on request and destroyed after the buffer has been
written to the log.
Passing a NULL string to the internal continuous log function triggers a write
of the buffer to the log.
BREAKING CHANGE: continuous log function is moved from log header to private
header.
From the kernel docs:
> -EINTR, -EAGAIN or -ERESTARTSYS, if the IOCTL should be restarted. This can
> either be due to a pending signal, or because the driver needs to completely
> bail out to recover from an exceptional situation like a GPU hang. From a
> userspace point of view all errors are treated equally.
We're not handling ERESTARTSYS because it's non-POSIX.
