escape_special_chars common function

Function: smarty_function_escape_special_chars
Purpose: used by other smarty functions to escape special chars except for already escaped ones

• author: Monte Ohrt <at ohrt dot com monte at ohrt dot com>
• usedby: smarty_function_html_checkboxes()
• usedby: smarty_function_html_options()
• usedby: smarty_function_html_radios()
• usedby: smarty_function_html_image()

• string $string