Fix a cornercase related to empty pass objects.

Fixes #1612
Testcase by: 	daghf

Conflicts:
	bin/varnishd/cache/cache_http1_deliver.c
Check for multiple Content-Length headers
Make it possible to receive resphdrs and respbody separately.

Always interpret C-L in decimal
Fix a couple of bugs for the detached rx*body functions.
Add a debug facility to force a WRW_Flush after the headers.

Conflicts:
	bin/varnishd/cache/cache_http1_deliver.c
Don't needlessly throw away Content-Length from backend, but assume
it is sane and pass it to streaming clients, provided we don't munge
the body along the way.

The ungzip'ed length stored as attribute has not been backported to
4.0, meaning on-the-fly ungzip will be delivered chunked.

Fixes: #1506

Conflicts:
	bin/varnishd/cache/cache_fetch.c