Code formats and cleanup
Text overlay on images.

1. Various code fixes
2. Added addText() method for text overlay
Better file operation

Use curl_file_create() by default, and define a fallback function while
it’s not available.
Utility functions update.

Addded validateDateTime() method.

The new getListRange() method is used for List-Range headers, designed
for partial collection retrieve.

Refined isCLI() and isURL() method.