A new fucntion, ttl_now(VRT_CTX), defines what "now" is when ttl
and age are calculated in various VCL subs. To sum up,

* Before a backend fetch on the client side (vcl_recv, vcl_hit,
  vcl_miss) we use t_req from the request. This is the significance
  in this commit, and fixes the bug demonstrated by r02555.vtc.
* On the backend side, most notably vcl_backend_responce, we keep
  the old "now" by simply using ctx->now.
* In vcl_deliver we use ctx->now, as before.

It was necessary to make all purges use t_req as their base time.
Then, to not break c00041.vtc it was necessary to change from ">="
to ">" in HSH_Lookup.

All VMODs that currently use HSH_purge must change to using
VRT_purge.

Fixes #2570

Related to #2570

This adds the VUS interface to libvarnish.

UDS code corresponding to VTCP_open() and VTCP_listen_on() is
implemented in the vtc code, since it will only ever be used
there.

instead of the other way around.

don't care about in libreadline.

Fixes #2550

which uses it.

Add a new sess_attr xport_priv which PROXY can use to keep state,
for instance for TLV attributes (see #2561)

After it was reported on #varnish that following the previous
example may lead to invalid response headers.

present in the desired workspace.

I'm a little bit uneasy about the tacit assumption that arguments
to VRT_String() now belong to VRT_String(), but we'll see how it
goes.

Fixes: #2132

VRT_CollectString() fail the VCL on overflows.

VRT_StringList() can be used in the rare and probably
non-existent cases where overflow-marking is undesired.

More work on vcl4.1 reserved words etc.

In vcl 4.1 symbol names are unique across all types & kinds.

We want it to be very explicit when non-default VCL versions
are used in test-cases.

For now, the only case this adds is a missing leading zero in the
day of the month of RFC822/RFC1123 - style http dates as in

		Fri, 2 Mar 2018 14:26:02 GMT (bad)
instead of	Fri, 02 Mar 2018 14:26:02 GMT (correct)

This is clearly invalid according to
https://tools.ietf.org/html/rfc7231#section-7.1.1.2
yet accepted by other webserver software.