Bigger hammer: panic() is similar to lbv_assert() but takes a struct sess *

Bigger hammer: panic() is similar to lbv_assert() but takes a struct sess *
from which it tries to extract as much useful information as possible before
it croaks.  The corresponding macro is spassert(), which assumes you have
a struct sess *sp in scope.  This should help a lot with bugs like #167
where important information is not available post-mortem (because sp->obj
points into the cache, which is not included in the core dump)

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2254 d4fa192b-c00b-0410-8231-f00ffab90ce4