(ctrl/node.dab) is constantly closed and re-opened for every non-locking read.
This is really slow across network file systems and unnecessary, so use a
similar optimization as the C++ sbbs_t class where the file can (and normally
is) left open across multiple consecutive reads.
Create/use opennodedat() function.
Uses the new CLOSE_OPEN_FILE() macro from xpdev/filewrap.h.