Fixes #2227

Please fill in the rest! This first draft has content that I know about:
* The return of hit-for-pass
* Changes in VRT and libvarnishapi

Make sure Age is always less than max-age

we should probably unify the '-' and CNUM case in vcc_expr4() some time

I wanted to update cache_fetch.dot to add the new transition from v_b_r
but I wasn't sure how the 304 case would be handled. The documentation
for built-in subs and the diagrams are also missing the fail transition.
The latter may deserve a special treatment.

do not send HTTP/2.0 headers to backends on pass from HTTP/2.0 clients

Previously, objcores to be removed remained flagged as referenced, so exp_mail_it
could change the flags again.

Fixes #2143

Instead of alphabetical, sort the options by category.

Pull -x out of the shadows, despite the RST markup, the output
satisfies "instruction at the point of need".

special mode arguments (-V, -x and later -C) break free early.

Expurge long tail content from the cache to keep the number of bans
below this value. 0 disables.

This is a safety net to avoid bad response times due to bans being
tested at lookup time. Setting a cutoff trades response time for cache
efficiency. The recommended value is proportional to
rate(bans_lurker_tests_tested) / n_objects while the ban lurker is
working, which is the number of bans the system can sustain. The
additional latency due to request ban testing is in the order of
ban_cutoff / rate(bans_lurker_tests_tested). For example, for
rate(bans_lurker_tests_tested) = 2M/s and a tolerable latency of
100ms, a good value for ban_cutoff may be 200K.

Merges #2131

object creation.

Fixes #1764

Revert a regressive nit from bdd84ffc which
has nothing to do with H/2

https://tools.ietf.org/html/rfc7231#section-5.1.1

   Requirements for servers:

   o  A server that receives a 100-continue expectation in an HTTP/1.0
      request MUST ignore that expectation.

So if Expect exists but is not "100-continue", we send the 417 irrespective
of the protocol version.

But if is is 100-continue and the protocol is HTTP/1.0 or below, we ignore
the header.

This lets you set or change environment variables in a test case.

Usage is

    setenv FOO "bar baz"

The above will set the environment variable $FOO to the value
provided. There is also an ``-ifunset`` argument which will only set the
value if the the environment variable does not already exist:

    setenv -ifunset FOO quux

The ifunset argument corresponds to passing zero as the third argument
to setenv(3).

agnostic function.