Introduce an osbuild API that can be used by the container to talk
to the osbuild host. It currently supports one method 'setup-stdio'
which should be used by the container to setup its standard input/
output so the stages can transparently do i/o with the osbuild host
via stdio.
The input data (args) is written to a temp-file backed buffer. The
output is either the host's stdout directly or another temp-file
backed buffer; the latter is re-opened (via /proc/self/fd) to get
another file-descriptor for the container, so in theory the host
and the container could do i/o to the same buffer independently.
