* Actually read CLI args in emscripten
* Fix fetchfs manifest parsing, increase download chunk size
The chunk size should probably be made a parameter in the future. The
larger chunk size trades longer hitches for fewer hitches.
* Add exec command driver and API functions for emscripten.
Under WASMFS, stdin/stdout can't be customized the way they can with
the JS FS implementation. Also, this approach frees up stdin/stdout
and simplifies interaction with the command interface for web embedders.
* fixup upload paths, show use of new emscripten cmd interface
* Add JS library function names to EXPORTS as well as EXPORTED_FUNCTIONS for older emsdk versions
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