Rework autocrap configuration for libedit/libreadline

As before, libedit is preferred over libreadline.

Fail configure if neither is found or if libedit includes are missing.
Require readline history support (as varnishadm needs it).

Previously, if only libreadline was found, all binaries were linked
against it. Now only binaries getting linked with LIBEDIT_LIBS will
get linked aginst readline if libedit is not found.

If configure succeeds, a build should not fail any longer due to
libedit/libreadline headers missing.

Fixes #1555
wrong kind of white space
Make vcl_hit{}'s access to obj.* happen through the packed string.

This bumps the vrt.h major version.
Don't populate obj->http any more.
Remove obj->http.

This shaves approx 100 bytes of stored objects, and is a major step
to avoiding storing absolute pointers in persistent stevedores.