Skip corrupt unlocker metadata in unlocker select and remove (closes #72)

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

README.md

@@ -198,7 +198,9 @@ Creates a new unlocker of the specified type:
 
 **DANGER**: Permanently removes an unlocker. Like Unix `rm`, this command
 does not ask for confirmation.  Cannot remove the last unlocker if the vault
-has secrets unless --force is used.
+has secrets unless --force is used. An unlocker directory that
+`secret unlocker list` skips with a warning, because its metadata cannot be
+read or parsed, is removed by the directory name the warning gives.
 - `--force, -f`: Force removal of last unlocker even if vault has secrets
 - **CRITICAL WARNING**: Without unlockers and without your mnemonic phrase,
   vault data will be PERMANENTLY INACCESSIBLE