Add a byte to the length field, but retain the 255 bytes restriction
for now.

Add symbolic names for the header fields and macros for multi-byte
fields, and uses these throughout.   This will make it easier next time.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@2287 d4fa192b-c00b-0410-8231-f00ffab90ce4