findUnlockerByID failed on the first unlocker directory whose metadata
could not be checked, read or parsed, so `secret unlocker select` and
`remove` failed when one sorted before the unlocker asked for. It now
skips such a directory with the warning ListUnlockers gives, through
the code both now share. A skipped directory is removed by its
directory name, with RemoveDirAtomic, and cannot be selected.
`unlocker remove` applies the last-unlocker check only when the ID is
that of the single listed unlocker, so removing a skipped directory is
never refused as removing the last one.
Model: opus-5-5
