People have been running into the maximum number of headers limitation
more and more lately, so increase this a little bit.  It is a
per-thread overhead, so it should not make any real difference when it
comes to memory consumption.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4422 d4fa192b-c00b-0410-8231-f00ffab90ce4