[xml] Character Sets supported

["Peter Jacobi" <pj walter-graphtek com>]

Hi Daniel,

To make sure I didn't overlooked something important:

1. Independant of whether iconv is used or other user supplied
character set handling is deployed, libxml2 will only ever support
those character encodings explicitely listed in encoding.h
    XML_CHAR_ENCODING_ERROR=   -1, /* No char encoding detected */
    XML_CHAR_ENCODING_NONE= 0, /* No char encoding detected */
    XML_CHAR_ENCODING_UTF8= 1, /* UTF-8 */
    XML_CHAR_ENCODING_UTF16LE=  2, /* UTF-16 little endian */
    XML_CHAR_ENCODING_UTF16BE=  3, /* UTF-16 big endian */
    XML_CHAR_ENCODING_UCS4LE=   4, /* UCS-4 little endian */
    XML_CHAR_ENCODING_UCS4BE=   5, /* UCS-4 big endian */
    XML_CHAR_ENCODING_EBCDIC=   6, /* EBCDIC uh! */
    XML_CHAR_ENCODING_UCS4_2143=7, /* UCS-4 unusual ordering */
    XML_CHAR_ENCODING_UCS4_3412=8, /* UCS-4 unusual ordering */
    XML_CHAR_ENCODING_UCS2= 9, /* UCS-2 */
    XML_CHAR_ENCODING_8859_1=   10,/* ISO-8859-1 ISO Latin 1 */
    XML_CHAR_ENCODING_8859_2=   11,/* ISO-8859-2 ISO Latin 2 */
    XML_CHAR_ENCODING_8859_3=   12,/* ISO-8859-3 */
    XML_CHAR_ENCODING_8859_4=   13,/* ISO-8859-4 */
    XML_CHAR_ENCODING_8859_5=   14,/* ISO-8859-5 */
    XML_CHAR_ENCODING_8859_6=   15,/* ISO-8859-6 */
    XML_CHAR_ENCODING_8859_7=   16,/* ISO-8859-7 */
    XML_CHAR_ENCODING_8859_8=   17,/* ISO-8859-8 */
    XML_CHAR_ENCODING_8859_9=   18,/* ISO-8859-9 */
    XML_CHAR_ENCODING_2022_JP=  19,/* ISO-2022-JP */
    XML_CHAR_ENCODING_SHIFT_JIS=20,/* Shift_JIS */
    XML_CHAR_ENCODING_EUC_JP=   21,/* EUC-JP */
    XML_CHAR_ENCODING_ASCII=    22 /* pure ASCII */

2. If assumption 1 is right, where do you see the preferred
extension mechanism:
a) only by adding to the  enum and the case statements
b) abonding the enum and assign encoding codes when
registering a character encoding
c) mix a) and b) (leave existing enum codes alone but
handle dynamically registered encodings in the default
clauses)

Regards,
Peter Jacobi