encoding: Detect truncated multi-byte sequences with ICU

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.

include/private/parser.h

@@ -140,4 +140,7 @@ XML_HIDDEN xmlChar *
 xmlExpandEntitiesInAttValue(xmlParserCtxtPtr ctxt, const xmlChar *str,
                             int normalize);
 
+XML_HIDDEN void
+xmlParserCheckEOF(xmlParserCtxtPtr ctxt, xmlParserErrors code);
+
 #endif /* XML_PARSER_H_PRIVATE__ */