Use a separate boolean to remember whether Content-Length was set.

Previous content_length == 0 was overloaded to mean that Content-Length
was set. But that was wrong when the Content-Length was actually 0.

The error message you got when running an HTTP proxy that received
0-length POSTs was
	POST request with no Content-Length.
New quake1-info script from Ulrik Haugen

Also added a version probe and match line.
http://seclists.org/nmap-dev/2014/q1/210