For early MUA spawners using lock-free outputs, we we need to
on the startq pipe to silence progress reporting. For
--augment users, we can start the MUA even earlier by
creating Maildirs in the pre-augment phase.
To improve progress reporting for non-MUA (or late-MUA)
spawners, we'll no longer blindly append "--compressed" to the
curl(1) command when POST-ing for the gzipped mboxrd.
Furthermore, we'll overload stringify ('""') in LeiCurl to
ensure the empty -d '' string shows up properly.
v2: fix startq waiting with --threads
mset_progress is never shown with early MUA spawning,
The plan is to still show progress when augmenting and
deduping. This fixes all local search cases.
A leftover debug bit is dropped, too