Tunable madvise for file storage

Adresses a new issue mentioned in #2004

Spotted by Foxik on #varnish @ irc.linpro.no.

Fixes #2011

Ref: #2008

We will need it if the body was consumed due to a call to
std.cache_req_body (VRB_Cache).

Analysis and test by daghf.

Fixes #1881.

pipe might receive a complete overhaul in the future but until then
make sure we pass this by default.

Prompted by simon on #varnish @ irc.linpro.no.

The previous attempt to address 1990 was only half-hearted, the fundamental issue
was that we could still fail to set even a 503 response code because we required
workspace to stringify it.

For well known status codes, we now keep constant strings together with the
default reason phrases and emit those.

Add a vtc for the 1990 case.

Fixes #1990

Better fix in the next commit

This reverts commit 83c5ead8.

Related to #2000.

Spotted by:	Régis Leroy

When out of workspace, we dumped the piece of memory at the ws free pointer,
which does not necessarily contain anything sensible in this situation.

From:       github::t-h-r-o-w-a-w-a-y

This is equivalent to synthetic but using the more general set keyword.

Before this commit it was only sent on pass'd requests, making
impossible to cache e.g. POST requests.

To keep the previous behaviour unset bereq.body if the method is GET,
which is normally true for misses.

Fixes #1927.

Only available in vcl_backend_fetch{}. If it is unset varnish will not
pass the request body to the backend.

This is in preparation for addressing #1927.

When handling a fetch error we may not even have had memory to store
the status code.

Fixes #1990

Relates to #1990

Fixes #1994.

soon need them.