... and document it

The size of 1 for the p array would not be required, it is a safety
measure only.

we only use one passed and one piped requests, so our servers should
only ever see two requests.

Motivated by a failure on fedora armv7hl, which I do not understand:

**   c1    === txreq -url /1 -hdr "pipe: true"
**** c1    txreq|GET /1 HTTP/1.1\r
**** c1    txreq|pipe: true\r
**** c1    txreq|Host: 127.0.0.1\r
**** c1    txreq|\r
**   c1    === rxresp
**** dT    3.832
**** c1    rxhdrlen = 0
---- c1    HTTP header is incomplete
**** dT    3.833

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