Show absolute time while waiting for lock instead of %, rework message

Showing a percentage for a timeout is pretty non-standard. Rework the
progress class so it can show an absolute progress (currently hardcoded
to use seconds as a unit). If there is a timeout (aka if it's not the
maximum long long unsigned -1llu), then show the timeout, otherwise
just count up seconds, e.g.

Waiting for cache lock: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 33842 (apt)... 1/120s

or

Waiting for cache lock: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 33842 (apt)... 1s

Also improve the error message to use "Waiting for cache lock: %s" instead of "... (%s)", as having
multiple sentences inside parenthesis is super weird, as is having two closing parens.

We pass the information via _config, as that's reasonably easy and avoids
ABI hackage. It also provides an interesting debugging tool for other
kinds of progress.

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