Implement custom log formats for varnishncsa

Make it possible to specify log formats for varnishncsa using -F.

We don't support arbitrary HTTP headers yet, and there are no docs yet
either.  Also missing is validation of the format when starting up,
rather than on the first line of the request.

Fixes #712
Fixes #485



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5489 d4fa192b-c00b-0410-8231-f00ffab90ce4