Use the Unicode replacement character for replacing unencodable characters into UTF-16

[Mattias Engdegård]

The attached patch makes sure that non-Unicode characters are replaced with 
U+FFFD REPLACEMENT CHARACTER instead of a space when converting to UTF-16. (The 
space is from all evidence a historical accident.)

This change is required for one possible solution of bug#42904. We can do 
without this patch, but it fixes a clear bug.

For some reason, unpaired surrogates aren't affected despite not being 
encodable in UTF-16 -- another bug, but not one addressed here.

0001-Use-Unicode-replacement-character-for-unencodable-UT.patch
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