Rather than store a skeleton IPX header in an mbuf hung off the SPX

Rather than store a skeleton IPX header in an mbuf hung off the SPX
PCB, simply embed it in the PCB, avoiding additional memory overhead,
memory allocation overhead, and removing one of the few remaining
uses of dtom() in the network stack.

Restore misplaced spx_ctlinput() from an earlier commit.

MFC after:	1 month