There were several issues in `secret_tool_action_lock()`:

- `g_autolist (GList)` isn't a correct type, as the list elements are
  `SecretCollection`s, not `GList`s
- Separately from that, the list didn't take ownership of the elements
  either in all cases
- We were leaking the `locked` and `context` variables

This commits just does away with all the g_auto* usage as it's the only
place in the code we're using it anyway, and just does all the freeing
at the end of the function.

Fixes: 015ea119 ("secret-tool: Add locking capabilities to secret tool")
Fixes: #89