Add parameter to configure maximum headers runtime parameter

When varnish receives more headers (from client or backend) than it is configured, then it refuses to process it and just returns 503 error with internal error of http format error.

There is runtime parameter http_max_hdr to deal with this limit (it defaults to 64), see: https://www.varnish-cache.org/docs/3.0/reference/varnishd.html#run-time-parameters

Sometimes it might be required to accept more headers than it is configured by default (for example in some installation of magento, when registering or placing an order).

This change allows to change this configuration.