WASMFS hard-codes stdin, stdout, stderr behavior

In the JS FS, it's possible to use `FS.init(stdin, stdout, stderr)` or `Module['stdin']` (&c) to customize the standard file handle behaviors. WASMFS does define `/dev/stdout` (and so on), but hard-codes their behavior to `emscripten_out`, `emscripten_err`, and reading a string from a browser prompt (`FS_stdin_getChar`). stdout and stderr are also always buffered.

Instead, they should use `Module['stdin']`, `Module['stdout']`, and `Module['stderr']` if defined, or some other mechanism should be used so a caller can wrap a process's input/output.