html.c: add html_url_arg

This function can be used to properly escape querystring parameter values.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

html.h

@@ -10,6 +10,7 @@ extern void html_status(int code, const char *msg, int more_headers);
 extern void html_txt(char *txt);
 extern void html_ntxt(int len, char *txt);
 extern void html_attr(char *txt);
+extern void html_url_arg(char *txt);
 extern void html_hidden(char *name, char *value);
 extern void html_option(char *value, char *text, char *selected_value);
 extern void html_link_open(char *url, char *title, char *class);