1. `po` nodes are exported too (plus edges).

* [INODE_EXPORT,NEW] Added exporting of edges between `po` nodes and between `p` and `po` nodes.
* [INODE_EXPORT,NEW] Added exporting of `po` nodes.
* [INODE_EXPORT,EXT] Rewrote the writing of inode labels using the newly rewritten, generic printing APIs.