Minor simplification
Make pipe-mode close the director/backend through the VDI api.
Always close directors through the VDI api.
We don't need a vsl copy any more.
Simplify even more.
Move HTTP1_Read() to http1_vfp, make it static and rename as appropriate.
More backend/http_conn cleanup
If we know an beresp.body is zero bytes long, zap any Content-Encoding: it
might have, it doesn't make any sense.

Fixes	#1602
Add test case for #1602
Also neuter do_gzip and do_gunzip if content length is zero