The "stdio" File objects (stdin, stdout, stderr) did not work on Windows,

because the stdout FILE* is a different address in jsexec than it is in sbbs.dll
(where the actual File I/O operations occurred). Refactored by passing the
stdio file descriptor (and open mode) to js_CreateFileObject rather than the
FILE* and using fdopen() to get a FILE* associated with the descriptor.
stdout.write() now works, for example.