eCAP: Return unknown body size for bodies with unknown body sizes.

eCAP: Return unknown body size for bodies with unknown body sizes.

Previously, unsigned -1 size was returned for those bodies.
Old adapters relying on that buggy behavior will now throw:

  !"attempt to use unknown libecap::BodySize size"

Such adapters should check libecap::BodySize::known() before
using the libecap::Body::bodySize() value.