OFString: Zero-terminate UTF-16 strings.

This partly reverts e8502c7.

The rationale behind this is that, on some OSes, native APIs (on e.g.
Windows) take UTF-16 strings that are zero-terminated.

However, forcing zero-termination for every string so that -[characters]
returns a zero-terminated UTF-32 string does not make sense. Therefore,
in the future, -[UTF32String] will be added, which will include
zero-termination. OFString subclasses can then just return their
internal representation if it includes a terminating zero or create a
copy that has a terminating zero if not.