Add column header for info fibres, and add tabs between fields

All other fields seem to closely mimic the output of info threads. I couldn't find a python API way to retrieve the inferior name, so that field (and others, like the names/values of function parameters) are missing.

One way to address this gap: we could just parse the output of bt and fill it in info fibres. But at the moment we just use the python API's functions/classes in inspecting things instead of parsing gdb output.