server/: Prepare an interface for multiple bulk-crypto transforms.

server/: Prepare an interface for multiple bulk-crypto transforms.

The current bulk-crypto transform is rather old-fashioned (though in
most formal senses secure).  It is neither as efficient as it could
be (in terms of overhead); nor does it provide privacy properties which
are as good as I'd like.  So it would be good to be able to replace it
with something better.

There's now a table of named transforms.  It only has one entry, `v0',
which is compatible with previous versions, but we now have all of the
machinery necessary to add new transforms later.

There are also some minor improvements to the tracing of cryptographic
details (notably: reporting the types of messages being encrypted and
decrypted); and an interface change for @ks_encrypt@ and @ks_decrypt@,
which are now allowed to corrupt @buf_u@ in the course of their
operation.