Move a Refused Target Aside Rather Than Removing It and Wait Only for a Reader

The two-step rename fallback removed the target before its second try,
and rename(2) reports the same errors for a source it cannot move, so a
refusal about the source cost the caller its existing file. The target
is moved aside and put back if the second step fails too.

The wait for a reader holding the target open was layered on the write
helper and ran on every platform and for every refusal, so a read-only
target on Windows cost half a second before failing anyway, while G-code
exports through rename_file() got no wait at all. It lives in
rename_file() now, on Windows only, and only for a target this process
could write. A failed config write clears the dirty flag as it always
did, so the idle handler does not repeat it on every tick.

The lock file is created readable and writable by every user, since
another user sharing the data dir has to open it read-write; the check
that the file behind the path is still the one opened runs at most every
few seconds rather than once per preset during a scan; the physical
printer loader reads under the lock; the stat headers join the existing
platform include block; and the utility tests keep their file's tag.

src/libslic3r/AppConfig.hpp

@@ -453,7 +453,7 @@ private:
 	MachineSettingMap											m_printer_settings;
 	// Writes the assembled config text, and on Windows its checksum and a backup copy. `checksum_source`
 	// is the text load() will verify, which for the JSON config ends before the trailing newline.
-	bool												write_config_file(const std::string &path, std::string body, const std::string &checksum_source);
+	void												write_config_file(const std::string &path, std::string body, const std::string &checksum_source);
 
 	bool														m_dirty;
 	// Original version found in the ini file before it was overwritten