Revert "Reset busyobj state to BOS_INVALID in VBO_GetBusyObj"

This reverts commit 0f90ee223daf97d4e6d83a78ae919a95e590bcca.

After looking at this again, I see that the busyobj is cleared as it
should as part of VBO_DerefBusyObj(). Reverting the patch.
Reflect new C: close default in vcl_pipe

We now set Connection: Close on all piped backend requests, to avoid some of
the debugging pain seen over the last 2-3 years.

Basic problem is that request backend routing done in vcl_recv would not be
honored for subsequent requests after one was piped.

This was, at least for new users, a rather nasty surprise.

Original change in commit 880c3c2.
Emphasise the values.
Note that directors stack.
Update purging docs for V4.
How to call purge from recv.
Directors can stack.
Error is gone.
Fix error syntax.
Update for V4 syntax.
Another error/synth.
Remove saint mode.
Remove saint mode references.
Remove references to vcl_error.
Updated to running.rst in users guide
Done. Important changes are consequently commented and all comments are marked with the string 'benc' for easy identification. There are some heavy refactorings done, that fundamentally changes content meaing.
minor update
Minor cleanups
Use pipe for chunked POST/PUT requests.

We don't know how to handle incoming chunked bodies currently, but the backend
might.
Squash some RST warnings