cbuf: make calling cbuf2buf on a cbuf by its owner an error
cbufp: refactor code for mapping cbufps
cbufp: new interface functions map_at and unmap_at

map_at is meant to support mapping a cbufp's memory buffer
directly into another component's VAS. unmap_at deletes the
mapping.
cbufp: add map_at and unmap_at to unit_cbufp