This line ain't too long to be broken.

And update the comment explaining what happens. This is to make it
apparent that a VFP has changed C-E in the stored object, so C-E
should not be changed by an IMS fetch, and a new ETag from IMS has
to be weakened. The flag is appropriate for use by any VFP that
needs to manipulate C-E this way (not just gzip).

Closes #2910

Thank you, @fgsch

Also add a duration argument type and the operators >, >=, < and <=
for use with it (besides the existing == and !=).

obj.ttl and obj.age are compared relative to the time the ban was
issued, as required by the fact that bans are evaluated some arbitrary
time after they are created. For this reason, the ban_dups parameter
has no effect on obj.ttl and obj.age bans, duplicates of bans using
these fields are never removed.

obj.grace and obj.keep are compared as absolute values.

In an effort to support The Proprietary Stevedore (tm) [or any other storage
engine supporting persistent bans - if any?], the existing ban serialisation
format is preserved and upgrades _should_ work seamlessly. But downgrades won't.

Using the BANS_HAS_* macros, we try to make the relationship between the ban
serialisation in cache_ban_build.c and access in cache_ban.c more obvious.

We now generate all fields and operators from table files as well as
the definition of allowed operators for fields. The user help text
about allowed operators is generated at init time.

The second vcl did not actually do anything useful because the second
client would hit a cached response.

as agreed on -commit.

Closes #2906

Refs 832d6da2

Fixes #2834

Fixes #2185

Test case by @fgsch

Thank you to phk for the improvement over the initial suggestion

When running really massive runs, "-j180 -n10000" kind of things,
the "rm -rf" of the tmpdir becomes the limiting factor.

The new -C option sends that int nice(1)'ed child process.

Two of them have no init step, and the third one is fail-safe.

And get a chance to catch vdp->fini() failures with the offending VDP
still present in the list of filters.

also when nothing has been received.

but also make it fail faster when appropriate

Apologies for the spam, I didn't think this would be a recurring
pattern.

Fixes: #2844

I caught one during a review and figured I might as well sweep the whole
tree, so this patch was created using Coccinelle and the following steps:

    $ cat >check_obj.cocci <<EOF
    @@
    expression obj, magic;
    @@

    if (obj != NULL) {
    - CHECK_OBJ_NOTNULL(obj, magic);
    + CHECK_OBJ(obj, magic);
    ...
    }
    EOF
    $ spatch --dir . --in-place --sp-file check_obj.cocci

This is my fourth semantic patch, I wouldn't mind checking them in to
have them handy in the repo to semi-automate code polish every now and
then. For example in a /tools/cocci directory. This way people could
study them or use them in their out-of-tree projects like VMODs or VUTs.