thread pointer-ref, and deref it on thread scheduling failure.

The code tried to deref the same pointer twice, which failed because
the VBO_DerefBusyobj() will clear the pointer when called. Separating
allows calling VBO_DerefBusyobj() for each of them.

Fixes: #1628

backslash is used for escaping in varnish-cli, so escape the backslash
to actually pass "\.png$".

Minor cosmetic changes while here. Discussed with Dag on Frankfurt.

Move explanation to its own section. Also mention the state options.

Because -0 == +0, the use of negative numbers as a distinction between
hits and misses in the recorded bucket history fails when there are
entries of index 0.

Fixes: #1623

Added two counters to VBE.
conn: concurrent connections to backend.
req: number requests sent to backend.

With this is possible to find out whether the request was a pass (or
hit-for-pass) in a single place.

This had the same effect as writing to beresp.uncacheble.

abandonned us.

Fixes	#1624

I'm not sure how valuable this test is but put it in the right place
until it's decided.

Fixes #1616

The varnish-counters man page looked so sad for the
counters without long descriptions. Not there yet, but a step
on the way.

In the long description we have sufficient room to
use the full word.

The sms_ counters were removed previously

Fixes #1611

Submitted by:	Muhammad Adeel via github

but do insist on it being "close" if we need that.

Test case by daghf

Fixes #1613

no threads available in the pool, fail the fetch and count it.

Fixes #1620
Fixes #1621

Right Thing instead.