Support for Java's "modified" UTF-8

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Java's standard library has routines that generate "modified" UTF-8 strings, which
are unreadable using the standard UTF-8 support in FLEXI-STREAMS.

The following link describes the differences, which are aimed at preventing NUL bytes
from being part of the data:

https://docs.oracle.com/javase/7/docs/api/java/io/DataInput.html#modified-utf-8