Make k5buf more flexible for binary data

Make k5buf more flexible for binary data

Remove the invariant that buf.data is always zero-terminated, to allow
marshalling of binary data into precisely allocated fixed regions.
Add k5_buf_cstring() to zero-terminate the buffer and retrieve the
data pointer.  Adjust all callers that build C strings appropriately.

Add a k5_buf_add_byte() convenience wrapper alongside the integer
marshalling wrappers.  Change k5_buf_init_fixed() to accept a void
pointer so it can more conveniently be used with uint8_t arrays.