parser: Rework encoding detection

Introduce XML_INPUT_HAS_ENCODING flag for xmlParserInput which is set
when xmlSwitchEncoding is called. The parser can use the flag to
reliably detect whether an encoding was already set via user override,
BOM or other auto-detection. In this case, the encoding declaration
won't be used to switch the encoding.

Before, an inscrutable mix of ctxt->charset, ctxt->input->encoding
and ctxt->input->buf->encoder was used.

Introduce private helper functions to switch encodings used by both the
XML and HTML parser:

- xmlDetectEncoding which skips over the BOM, allowing to remove the
  BOM checks from other encoding functions.
- xmlSetDeclaredEncoding, replacing htmlCheckEncodingDirect, which warns
  about encoding mismatches.

If users override the encoding, store the declared instead of the actual
encoding in xmlDoc. In this case, the actual encoding is known and the
raw value from the doc is more useful.

Also use the input flags to store the ISO-8859-1 fallback state.
Restrict the fallback to cases where no encoding was specified. (The
fallback is only useful in recovery mode and these days broken UTF-8 is
probably more likely than ISO-8859-1, so it might eventually be removed
completely.)

The 'charset' member of xmlParserCtxt is now unused. The 'encoding'
member of xmlParserInput is now unused.

The 'standalone' member of xmlParserInput is renamed to 'flags'.

A new parser state XML_PARSER_XML_DECL is added for the push parser.

include/libxml/tree.h

@@ -573,12 +573,11 @@ struct _xmlDoc {
     struct _xmlDtd  *extSubset;	/* the document external subset */
     struct _xmlNs   *oldNs;	/* Global namespace, the old way */
     const xmlChar  *version;	/* the XML version string */
-    const xmlChar  *encoding;   /* external initial encoding, if any */
+    const xmlChar  *encoding;   /* encoding from XML declaration, if any */
     void           *ids;        /* Hash table for ID attributes if any */
     void           *refs;       /* Hash table for IDREFs attributes if any */
     const xmlChar  *URL;	/* The URI for that document */
-    int             charset;    /* Internal flag for charset handling,
-				   actually an xmlCharEncoding */
+    int             charset;    /* unused */
     struct _xmlDict *dict;      /* dict used to allocate names or NULL */
     void           *psvi;	/* for type/PSVI information */
     int             parseFlags;	/* set of xmlParserOption used to parse the