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.
`toString()` will only be used when `util.isBuffer(data) === true`. Thus `toString()` is always expected.