Keep the Sweep Out of the Write Helper and Honour a Read-Only Target

Sweeping the target's directory from inside write_file_atomically() made
every settings export into a user's folder delete their own numbered
files that matched the older config temporary form, and cost a directory
walk per save. The helper writes its target and nothing else; the user
preset scan, the bundle metadata reads and AppConfig::load() sweep the
directories the application owns, once, while they hold the lock.

Replacing a file needs only a writable directory, so a preset or config
the user made read-only was overwritten where the in-place write used to
fail; such a target is refused before anything is written. The CLI's
load_if_exists() takes no lock and creates no lock file, since the CLI
never saves. The lock guard holds the slot mutex through a unique_lock,
so an exception during construction cannot leave the slot locked for
good, and it counts its entry last so a throw leaves the slot as found;
the cool-down after a failed open is set where the failure is seen.

The cloud agent's sync state and secret fallback file and the 3DPrinterOS
session file wrote through a fixed ".tmp" name with a non-Unicode stream;
they call the helper. The vendor cache failure test makes the cache
read-only, which the helper refuses on every platform, and the utility
tests carry the PascalCase tag the test rules ask for.

src/libslic3r/AppConfig.hpp

@@ -121,8 +121,9 @@ public:
 
 	// Load the slic3r.ini from a user profile directory (or a datadir, if configured).
 	// Return an error string, or an empty string on success.
-	std::string         load();
+	std::string         load(bool read_only = false);
 	// Treat a missing config as default state; otherwise load it normally.
+	// The CLI's load: it never saves, so it takes no lock and creates no lock file.
 	std::string         load_if_exists();
 	// Store the slic3r.ini into a user profile directory (or a datadir, if configured).
 	void 			   	save();