parser: Rework EBCDIC code page detection

To detect EBCDIC code pages, we used to switch the encoding twice and
had to be very careful not to decode data after the XML declaration
before the second switch. This relied on a hard-coded expected size of
the XML declaration and was complicated and unreliable.

Now we convert the first 200 bytes to EBCDIC-US and parse the encoding
declaration manually.

include/libxml/encoding.h

@@ -203,6 +203,7 @@ XMLPUBFUN int
 	xmlCharEncInFunc		(xmlCharEncodingHandler *handler,
 					 xmlBufferPtr out,
 					 xmlBufferPtr in);
+XML_DEPRECATED
 XMLPUBFUN int
 	xmlCharEncFirstLine		(xmlCharEncodingHandler *handler,
 					 xmlBufferPtr out,