Previously we would ignore errors to iterate the request body into
oblivion in VRB_Ignore(), keeping the connection open. This opens an
out-of-sync vulnerability on H/1 connections.

This patch tests the status of the request body in VRB_Ignore(), marking
the request failed and that it should be closed on errors.

When receiving H/2 data frames, make sure to take the advertised content
length into account, and fail appropriately if the combined sum of the
data frames does not match the content length.

The change to VTCP_Check() in 58a21da7
broke expect_close in varnishtest.

Varnish packages are always built using a release tarball. This change
enables sending build URL for tarball to be downloaded from and SHA256 sum
to validate against as build parameters.

If a specific commit is used and the tarball is created for the build
step, the .is_weekly flag file will always be set, causing the packages to
be dated.

Restructured so that:

* 'Upgrading' is limited to work that has to be done to upgrade from
  a current deployment to the new version.

* 'Changes' is a comprehensive, user-level description of changes and
  new features.

Conflicts:
	doc/sphinx/whats-new/index.rst

Committer Edit:
	Added Copyright header by example of
	fe8cfe84 and
	93439884

Noted by Simon Vikström, thank you

Closes #3551

The order matches the order of commits, except where I thought it made
more sense to join topics into contiguous sections.

We can incorrectly reference resp.reason from other sources when
jumping into vcl_synth. This also covers passing in a reason in
vcl_backend_error.

Provide a set of predictable subroutines that will be called by the
built-in vcl. Only transactional built-in subroutines are concerned,
which excludes vcl_init and vcl_fini.

Closes #3548

Better diff with the --word-diff --ignore-all-space options.

When it was in its own file it worked fine...

And fix the h2_req VDP error handling as per the VDP contract.

Test case inspired by Simon. Since this is one of those test cases that
explicitly mix two features I wasn't sure whether I wanted to make this
an h2 test case or a range test case. Since this was ultimately a range
bug I decided to register it in a range test case.

It's not obvious what should have been authoritative here. The range VDP
was rightfully latching an error via SC_RANGE_SHORT that is defined as an
error-type session close reason, but VDP_DeliverObj() doesn't take that
into account. While SC_RANGE_SHORT isn't a session/protocol error for h2
but rather a stream error it is not obvious what VDP_DeliverObj() should
do in the absence of a negative retval and the presence of a non-null
sess_close.

Maybe another way could be to turn enum sess_close into a struct and
embed http1 and h2 specificities directly in struct fields. We already
have somewhat structured information in the sess_close.h table.

Refs 03f71c6e

to support more realistic practical test cases like JWTs

Only copy the host header if needed

It's one more indirection than vcl_recv because the same logic applies
to multiple cases, so we first have subs for the different conditions
and they all call the shared vcl_beresp_hitmiss sub.

The vcl_beresp_hitmiss sub returns on purpose, that doesn't change the
default VCL behavior and makes it more usable from VCL code:

    if (some user condition applies) {
            call vcl_beresp_hitmiss;
    }

No need to force all call sites to return(deliver) when the desired
outcome is explicit. Extracting this sub also enables VCL authors to
only tweak the hitmiss TTL in one place.

For better compliance, we might also introduce a vcl_beresp_hitpass sub
for some of the built-in cases where a hit-for-miss object might be less
appropriate. The compliance effort will however change some semantics
and is outside of the scope of this split.