UTF-8 string support:

updated utf8_strtrunc()
added utf8_substr(), utf8_strlen(), utf8_strpos(), utf8_strrpos()
added helpers utf8_char2byte_pos(), utf8_byte2char_pos()


git-svn-id: https://svn.typo3.org/TYPO3v4/Core/trunk@163 709f56b5-9817-0410-a4d7-c38de5d9e867