This adds some new functions: sshbuf_from(), sshbuf_fromb() and sshbuf_froms()
that create new buffers from a read-only blob, the contents of a buffer and
the contents of a string in a buffer respectively. In all cases the memory
contents are not copied - the new buffer, which is read-only, just refers to
the original location in memory.
removed fatal()
buffer_ => sshbuf_
changed lookup of identies to return a fully-deserialised list of keys
removed AuthenticationConnection abstration; all operations now use plain socket