Vtable objects only need to be globally visible throughout the code if
they're used directly in some interchangeable way, e.g. by passing
them to a constructor like cipher_new that's the same for all
implementations of the vtable, or by directly looking up public data
fields in the vtable itself.
But the BPPs are never used like that: each BPP has its own
constructor function with a different type signature, so the BPP types
are not interchangeable in any way _before_ an instance of one has
been constructed. Hence, their vtable objects don't need external
linkage.
Getting it out of the overgrown ssh.c is worthwhile in itself! But
there are other benefits of this reorganisation too.
One is that I get to remove ssh->current_incoming_data_fn, because now
_all_ incoming network data is handled by whatever the current BPP is.
So now we only indirect through the BPP, not through some other
preliminary function pointer _and_ the BPP.
Another is that all _outgoing_ network data is now handled centrally,
including our outgoing version string - which means that a hex dump of
that string now shows up in the raw-data log file, from which it was
previously conspicuous by its absence.
