For backend requests, we do not have the thread local storage of the
client thread available, so trq.req will normally be (nil).

Dump the busyobj back pointers to req/sess/wrk to maximize the
usefulness of panics.

Also reorder the panic by struct busyobj and add some more members
which may or may not be useful (but when we find out, it is likely
too late).

Also move the vmod list dump to the top level.

Noticed when trying to understand if #2667 was also a stack overflow
in the context of #2817