Storing the node file (node.dab) descriptor in the scfg_t was a "bad idea" {tm}

The scfg_t instances are often shared between threads and we can't really share
file descriptors between threads, so we ended up with file descriptor leaks and
race conditions.

Instead, define/allocate/free a new js_system_private_t where the scfg_t* and
the nodefile (descriptor) are kept - per "system" instance.