late winter cleanup

shame on me for this commit.

- add _cleanup_free_ for simple allocations, use it.
- remove redundant fields from archive_conv and repo_t
update: always display transfer stats when we do real work
update: avoid ignoring error from add_repo_download in completion handler
update: tighten scoping
code reuse
command_not_found.bash: Avoid call to gettext

This never actually worked, since the domain was never set for gettext.
It also aids in triggering a bug in bash:

http://lists.gnu.org/archive/html/bug-bash/2013-12/msg00088.html
update: simplify prepare_url