Give SLT_Begin and SLT_Link a third field "why", and normalise the
first field to the type of entity.

Martin will commit corresponding and necessary changes to the
API in a matter of milliseconds or possibly hours or days, so
minor weirdness from varnishlog/ncsa can be expected for a brief
amount of time.

Testcase l00000 & l00001 are also broken until the API catches up.