VIP21: Avoid modified Zlib

See ticket #2512.

It is desirable to not carry a modified Zlib around, but the changes to avoid this are not trivial.

The only way to know the start/stop bit locations is if you use the right FLUSH operations when compressing and record them out of band.

That means that the present 'testgzip' step will have to be replaced by gunzip and gzip-with-flushes, for ESI processing to work with a unmodified Zlib.

Since ESI is always available in varnishd, we would have to do that with all gzip’ed contents, on the off-chance that some VCL loaded later starts using ESI and I do not feel that changing ESI to a startup-enabled feature is reasonable cost.

The way to get this box ticked is therefore to get our patch to zlib accepted upstream, and this has been rejected twice already I believe.