VCL control over Content-Length: 0 vs. no Content-Length

[nigoroll]

Now that the original issue has been closed as fixed (rightly so), this ticket is to get to a decision about how to add VCL control over whether or not to send Content-Length for an empty body.

I stick to my previous suggestion:

• Allow unset x.http.Content-Length and set x.http.Content-Length = "0".
• Both imply unset x.body.

Originally posted by @nigoroll in #4331

[nigoroll]

bugwash: yes, we want this, if the syntax check for "can only set to "0" can be implemented at compile time)