Add a check to avoid doing ESI parsing of objects that do not look

Add a check to avoid doing ESI parsing of objects that do not look
like XML.  Our definition of "looks like XML" is that the first
non-white-space character is '<'.

Add a new parameter "esi_syntax" with bits to steer the ESI parser.

Use the first bit to disable the "looks like XML" check.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3016 d4fa192b-c00b-0410-8231-f00ffab90ce4