This adds a reference mechanism to struct sshbuf, making parent buffers to
those created with buffer_froms() and buffer_fromb() read-only until the
child buffer is freed. It also delays freeing of the parent buffer until
all child buffers have freed.
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.
