Improve debbugs menu and buffer name

Improve debbugs menu and buffer name

* packages/debbugs/debbugs-gnu.el (debbugs-gnu):
Use `unwind-protect', in order to guarantee final cleanup.
(debbugs-gnu-show-reports): Make the buffer name depending on
query.  Delete default package name from `words', instead of
hard-coded "emacs".
(debbugs-gnu-manual): New defun.
(debbugs-gnu-mode-map): Add "Debbugs Manual" to menu.