examples: fix more potential resource leaks, and more

examples: fix more potential resource leaks, and more

Also:
- delete dead code.
- sync `http2-download.c` and `http2-upload.c` sources.
- simplessl: fix constant expression.
- simplessl: avoid `expression is constant` VS2010 warning, drop pragma.
- replace large stack buffers with dynamic allocation.
- http2-download: fix to fill transfer number.

Some of these were pointed out by TIOBE scanner via Coverity 2025.3.0.

Closes #19292