PAN_dump_struct2() is now renamed to PAN__DumpStruct() following the
guidelines from apispaces.rst in addition to growing a track argument.
This should allow dumping structures that we know ought to be dumped
only once, in case they would be numerous.

There seems to be no good reason to expose it to VMODs yet since they
have no integration point with the panic output, so those definitions
can be confined in cache_varnishd.h for now.

Better diff with the --ignore-all-space option.