HTTP header munging part (N of M)

HTTP header munging part (N of M)

NB: Only pass mode (lightly) tested right now.

Give up on the three element array per header and use a two element struct
instead, it reduces obfuscation and removes risk of pointer fandango.

Introduce #defined filtercontrol in http_headers.h, use them in a new
field.  Only Pass is there for now.

Use the http-workspace for building headers instead of sbuf.

Move uiovec handling to cache_pool.c where it more naturally belongs
and so we can use it on both backends and sessions.

Add http header munging functiosn for copying, printf'ing, filtering and
writing headers.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@533 d4fa192b-c00b-0410-8231-f00ffab90ce4