In the next patches we are going to teach _selcase to support both
external and inplace data. Before that let's do a couple of preparatory
things.

This patch: rename .data -> .ptxrx . The new name is more clear:

- "p" prefix aligns with other libgolang style, e.g. ptx or prx.
- "txrx" suffix says that this is used for both send and recv.

Just plain renaming, nothing else in this patch.