Move the objindex pointer and length to the segment table, having it
in the segment signatures is just a waste of disk-access on startup.

Put empty signatures at segment head, objidx and segment tail.

Align allocations to 16 bytes boundaries, for cache and hexdump
convenience.

Discard unsused segments on close.



git-svn-id: http://www.varnish-cache.org/svn/trunk@4279 d4fa192b-c00b-0410-8231-f00ffab90ce4