Now that we have the vpi ref as set by VPI_Count() available via struct
wrk_vpi, we can dump our position in VCL at the time of the panic.

Additional changes:

* move handling info from pan_wrk() to the new VPI_Panic()
* output the source index of vcl sources with the name in
  VCL_Panic()

The additional panic section looks like this:

vpi = 0x7f5478be93b0 {
  handling (VCL::return) = 0x0 (none),
  ref = 6,
  vpi_ref = 0x7f54753fb530 {
    source = 0 (\"<vcl.inline>\"),
    offset = 365,
    line = 20,
    pos = 25,
    src = \"vtc.panic(\\\"Had Panic header: \\\" + resp.ht[...]\"
  },
},

The source line is truncated (and truncation denoted by [...]) if longer
than 40 characters.

If the in-memory vcl data source string looks truncated, tok = "..." may
be output instead of src "...".