fs: Add file descriptor support to *File() funcs

These changes affect the following functions and their synchronous counterparts:

 * `fs.readFile()`
 * `fs.writeFile()`
 * `fs.appendFile()`

If the first parameter is a `uint >= 0`, it is treated as a file descriptor. In all other cases, the original implementation is used to ensure backwards compatibility. File descriptor ownership is never taken from the user.

The documentation was adjusted to reflect these API changes. A note was added to make the user aware of file descriptor ownership and the conditions under which a file descriptor can be used by each of these functions. The signature notation was updated to reflect the style used by more recent documentation.

Tests were extended to test for file descriptor parameters under the conditions noted in the relevant documentation.
Assuming fs.close() calls close(2), it cannot fail to release the fd. The error returns from close are purely advisory. Linux documents only EBADF (which isn't a leak, obviously), EIO (which is a delayed error from a prev i/o operation, interesting, but not a leak, and only NFS, I think), and EINTR, of which the following note applies:

```
Note that the return value should only  be
used  for  diagnostics.   In  particular close() should not be retried after an EINTR since this may
cause a reused descriptor from another thread to be closed.
```
- man 2 close