specific wait/exit code.

This is a compromise discussed at VDD15Q1 after a previous suggestion
to expose the 304 response status directly to VCL.

This way, VCL can differenciate between 200 and 304 responses, but simpler
processing logic just checking for status 200 can be preserved where
differenciating is not required.

A few things still remaining.

Fix off-by-one error just introduced for .initial setup.

the .initial setup.

(more precise if a header is set more then once or overwritten).

Fixes #1683

	varnish ${id} -expect !${vsc-identifier}

detailed probing information.

Retire the "debug.health" CLI command.

entire level of indirection in the backend probe code.