Except that the old default value replaces the maximum one. Aligning
with the literal maximum value for the underlying HTTP/2 setting breaks
32bit builds because the byte tweaks take a detour via ssize_t. When it
casts to uintmax_t the MSB is propagated all the way, triggering the
following error at build time:

> 4294967295b is too large for this architecture.

Instead of fighting a tweak that is clearly wrong, grant h2 clients a
maximum of 2GB of uncompressed headers (instead of 4GB) that will never
happen, because h2 is overall much wronger.

This parameter has a new role that consists in interrupting connections
when decoding an HPACK block leads to a header list so large that the
client must be stopped.

By default, too large is 150% of http_req_size.

Since http_req_size was already established for this purpose, and is
now enforced for h2 traffic, it should naturally become the basis for
the MAX_HEADER_LIST_SIZE setting in the initial SETTINGS frame sent
to clients.

The h2_max_header_list_size parameter will grow a new purpose.

With the exception of h2_max_header_list_size that is not advertised as
such despite being ent as part of the initial SETTINGS frame. The same
parameter also sees its default and maximum values updated to 2^32-1.

This is based on this sentence from rfc9113:

> The initial value of this setting is unlimited.

This aligns the h2_max_header_list_size parameter with the values set in
h2_settings.h for MAX_HEADER_LIST_SIZE.

For the sole purpose of having these limits tested in a single place.

Fixes #3709
Closes #3892

Refs #3709

It does a first pass on header names and values, and only logs errors,
so the signature is updated accordingly and the call site is moved into
h2h_addhdr().

Better diff with the --ignore-all-space option.

It became explicit in rfc9113:

> The same pseudo-header field name MUST NOT appear more than once in a
> field block.

While at it, the duplicate pseudo-header error can be consolidated in a
single location instead of adding one more branch.

Instead of passing both a decoder and individual decoder fields, the
signature for h2h_addhdr() changed to only take the decoder. The order
of parameters is destination first, then the source following the
calling conventon of functions like memcpy().

Internally the function is reorganized with a bunch of txt variables to
keep track of the header being added, its name and value. In addition to
clarity, this also helps improve safety and correctness.

For example the :authority pseudo-header name is erased in place to turn
it into a regular host header, but having a dedicated txt for the header
name allows its preservation.

The extra space before the colon looked uncanny. The rest is just code
indentation improvements.

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

When a stale object is soft-purged, the time until the object expires
should not be reset, as repeated soft-purges could keep the object
around indefinitely.

This commit introduces EXP_Reduce(), a function to reduce object timers.
The goal is to provide a function better suited to soft-purging objects,
as EXP_Rearm() has some non-obvious disadvantages when used or this
purpose.

When EXP_Rearm() is used to soft-purge an object by setting its TTL to
0, the expiry is effectively reset to the start of the objects grace
period. This happens because the object TTL includes a time delta
between object insertion time (oc->t_origin) and now. The result is that
a soft-purge extends the lifetime of an already stale object, and
repeated soft-purges can keep the object away from expiry indefinitely.

The EXP_Reduce() function is better suited for soft-purging, as it only
updates an objects TTL if it would reduce the objects lifetime. The
function also has facilities for reducing grace and keep.

If a format never matches anything, the 4294967296th transaction
proccessed by varnishncsa will wrap its generation around to zero,
be considered a match, and let vsb_fcat() pass a null string to
VSB_quote().

Better diff with the --word-diff --word-diff-regex=. options.

Releasing 7.5.0

Before the h2 frame dispatch we only need to check that coming from a
HEADERS frame the very next frame is a CONTINUATION, when the HPACK
block didn't fit in the former.

The CONTINUATION dispatch will then make the stream consistency check.

When a stream times out waiting for window credits, and all the other
streams are broke, declare the whole connection bankrupt.

It replaces idle_send_timeout when the stream is waiting for window
credits before sending more DATA frames. The idle_send_timeout will
still apply to individual writes to the socket, but triggering it is
considered a failure condition (this was already the case).

The two loops are merged into a single one to better deal with the
lack of ordering guarantees of request vs connection stream control
flow window crediting.

Refs #2980

This parameter needs to be somewhat high because web browsers may for
example only credit streams for image resources just enough to parse
metadata like width and height of a picture in order to perform layout
computations and fetch resources more sensitive to latency before the
effective image payloads.

The new meaning for h2->sess->t_idle is now the time since the last
complete frame, except before the first frame where it corresponds to
the creation of the session.

And after issuing or receiving a goaway, stop processing frames when
there are no streams left.

Initially submitted as part of listen sockets management.

Refs #3959