nmbug-status: Consolidate functions and main code

The definitions of Thread, output_with_separator, and print_view were
between the main argparse and view-printing code.  Group them together
with our existing read_config at the top of the module, which makes
for easier reading in the main section.

I also:

* Made 'headers' a print_view argument instead of a module-level
  global.  The list -> tuple conversion avoids having a mutable
  default argument, which makes some people jumpy ;).

* Made 'db' a print_view argument instead of relying on the global
  namespace to access it from print_view.