This function returns the content length and puts range indices in
lo and hi arguments. A content length of -1 used to be interpreted
as something to ignore, but there is a case where the content length
may be unknown.

Since we can't represent a zero-length range, because both bounds are
inclusive, zero now denotes the lack of content-range header.

Unknown range units are treated as errors as they wouldn't pass the
busyobj check for the range header, even for pass transactions. The
only way to use "extension" range units is to turn http_range_support
off.

The calling convention for http_GetContentRange() remains otherwise
the same, and for good measure http_GetContentLength() received a
similar description.

Fixes varnishcache#4089