Improve Gnus ephemeral bug group browsing

Improve Gnus ephemeral bug group browsing

* doc/misc/gnus.texi (Foreign Groups): Update description of
gnus-read-ephemeral-emacs-bug-group for multiple bug
IDs (bug#11961).

* lisp/gnus/gnus-group.el (gnus-bug-group-download-format-alist):
Use HTTPS for Debian's bug tracker.
(gnus-group--read-bug-ids): New function for reading multiple bug
IDs in the minibuffer.  Improves on previous brittle approach of
word-at-point -> read-string -> string-to-number by a) defaulting to
the more accurate bug-reference-bug-regexp or number-at-point
without using the intrusive INITIAL-INPUT argument, and b) not
attempting to parse bug IDs.
(gnus-read-ephemeral-bug-group): Use it.  Extend docstring and
commentary.  Fix handling of multiple bug IDs as either numbers or
strings.  Hoist some string consing out of inner loop.  Delete
temporary file even on error.  Throw more informative error when
url-insert-file-contents successfully returns an error (bug#36433).
(gnus-read-ephemeral-debian-bug-group): Use gnus-group--read-bug-ids
and fix docstring for multiple bug IDs.  Accept optional WINDOW-CONF
like other ephemeral bug group commands.
(gnus-read-ephemeral-emacs-bug-group): Use gnus-group--read-bug-ids
and fix string/numeric ID conversions.  Try loading debbugs-gnu
before testing for debbugs-gnu-summary-mode (bug#36433).