FT_STRINGZPAD is for null-*padded* strings, where the field is in an
area of specified length, and, if the string is shorter than that
length, all bytes past the end of the string are NULs.
FT_STRINGZTRUNC is for null-*truncated* strings, where the field is in
an area of specified length and, if the string is shorter than that
length, there's a null character (which might be more than one byte, for
UCS-2, UTF-16, or UTF-32), and anything after that is not guaranteed to
have any particular value.
Use IS_FT_STRING() in some places rather than enumerating all the string
types, so that those places get automatically changed if the set of
string types changes.
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