Refactor buffers; implement descriptors.
'buf_t' is now an opaque type defined in buffers.c . Router descriptors now include all keys; routers generate keys as needed on startup (in a newly defined "data directory"), and generate their own descriptors. Descriptors are now self-signed. Implementation is not complete: descriptors are never published; and upon receiving a descriptor, the directory doesn't do anything with it. At least "routers.or" and orkeygen are now obsolete, BTW. svn:r483