Add command line parameter --no-heap-copy that optimizes for small memory footprint and fread() performance over the default behavior that copied VFS to HEAP, that is designed for mmap() performance.

Adjust MEMFS node contentMode enum to reflect whether content is off the main HEAP or not. Note that this enum is not much used, so this has little effect. Add browser tests to check that fread() and mmap() work with and without --no-heap-copy.