radv: add a secure_compile_open_fifo_fds() helper

radv: add a secure_compile_open_fifo_fds() helper

This will be used to create a communication pipe between the user
facing device and a freshly forked (per pipeline compile) slim copy
of that device.

We can't use pipe() here because the fork will not be a direct fork
of the user facing process. Instead we use a previously forked
copy of the process that was forked at device creation in order to
reduce the resources required for the fork and avoid performance
issues.

Fixes: cff53da3748d ("radv: enable secure compile support")