flush_active() can be called multiple times from logbuffer_flush(), and
so can logbuffer_ref()

so far, this has only been seen with clang 10.0.0-4ubuntu1

Using different compilers never disappoints...

ioerr_log and allocerr_log have appropriate NOTEs attached.

I think statistics are good at this point and timeouts do not sound like
a good idea to me any more: What are we going to do when a timeout is
hit?

Spotted by Coverity, CID#486184

Closes #46

https://github.com/varnishcache/varnish-cache/commit/dcaf616c66d93de69735237967cc091fa490bb93
removed the option to specifically build a single VSC header.

Closes #56

... controlled by limiting parameters, which can also be used, in the
worst case, to deactivate the feature.

Closes #54
Closes !3 (implemented somehow similarly)

The potential impact of changing this parameter to a wrong value is too
high, so we better rely on changes to automatically adjust it (coming
up).

Ref #54

Each fellow storage reserves a minimum amount of memory.

Closes #51

this wrong line counteracted fdb260e2 :(

Part of the solution to #54

It does not make sense to have it larger than the logbuffer itself.

Because the logbuffer can be larger than the threshold, we need to flush
also for n >= thr.

Fixes 7e986ecc

Motivated by #45

Ref #55