Unlike iconv or the internal converters, ICU consumes truncated multi-
byte sequences at the end of an input buffer. We currently check for a
non-empty raw input buffer to detect truncated sequences, so this fails
with ICU.
It might be possible to inspect the pivot buffer pointers, but it seems
cleaner to implement a `flush` flag for some encoding and I/O functions.
After flushing, we can check for U_TRUNCATED_CHAR_FOUND with ICU, or
detect remaining input with other converters.
Also fix detection of truncated sequences for HTML, XML content and
DTDs with iconv.
69b83bb68e