We can directly use vmod-purge now.

This module offers a finer-grained control over purges, in the old
pre-vcl 4.0 style. For that, the signatures of VRT_purge and HSH_Purge
were updated to inform about the number of rearmed objects.

Fixes #2403.

This one is for h2 streams, so the thread pools needed to learn about
"stream" tasks. To properly test this counter, thread_pool_timeout is
now disregarded when the vtc_mode debug flag is set. A minimum value
of 10s is unrealistic for the test suite, it is now hard-coded to 0.5s
when ran through varnishtest.

A client can safely retry the request because no application logic is
involved when no worker can enter the VCL state machine. At this point
the HPACK block was properly processed, so Varnish remains in sync with
the client and may process future streams.

Fixes #2311

This patch fixes the queuing logic of tasks. Depending on the various
thread pool parameters, it could exceed up to `thread_pool_max` tasks
in the queue.

Fixes #2409.

Remove the unmap-handling from vsm_refresh_set.

Revert bdd70290 "Always munmap(2) segments when we delete them."

	#include <sys/mman.h>
Move them back.

Related to #2305

Spotted by:	fgs

to prevent recursion.

Spotted by:	fgs

Spotted by:	fgs

Instead of having a single mmap(2)'ed file inside which we allocate
the necessary chunks, put each chunk in its own file and use two index
files (MGR/CHILD) to keep track of them.

Major benefits:

* You don't need to guess how much VSM space you need at startup.

* VSM chunk Identifier (and therefore VSC counter names) are unlimited length.

Minor benefits:

* VSM API has simpler and explainable semantics now.

* The bogo-you-didnt-guess-right-malloc-VSM allocations are gone.

* -t argument moves up/down to VSM level and works the same through-out.

* VSM/VUT utils now stay attached even if the MGT process restarts.

Some relatively minor adjustments are still to come before the VSM API is frozen
and the .map file finalized for the release.

Previously, we could run out of stack handling stack overflows, leaving
users with unspecific SIGSEGV crashes and no panic message.

By providing a single alternative stack exclusively for SIGSEGV handling
where sigaltstack() is available, we increase chances for our signal handler
to finish successfully.

In particular, this will make it easier to diagnose stack overflows by
comparing the failing address with the stack info from the panic output.

This could be further improved by giving advise to increase thread_pool_stack
if si_addr is near the stack boundaries.

c00057.vtc now triggers a stack overflow instead of raising a SIGSEGV.

Merges #2396

it was already used for more than SIGSEGV, so we should output the actual
signal description.

For HitMiss and HitPass, only show the remaining TTL. Seeing these
values should help debugging of timing-sensitive transactions. The
grace and keep periods are absolute, but the TTL is the remaining
time relative to the current request. Same as obj.{ttl,grace,keep}
variables in VCL.