Fix my breakage to the low-level hardware sync drivers brought by the
inclusion of VJ compression into sppp.

Now, instead of the need to include this and that and everything plus
the kitchensink in each of those drivers, struct sppp uses struct
slcompress as an opaque structure only referenced by a pointer. The
actual structure is then malloced at initialization time.

While i was at it, also fixed a bug where received VJ packets would only
be recognized if INET6 was defined.