html: make all strings 'const char *'

None of the html_* functions modify their argument, so they can all be
'const char *' instead of a simple 'char *'.  This removes the need to
cast (or copy) when trying to print a const string.

Signed-off-by: Mark Lodato <lodatom@gmail.com>

html.h

@@ -7,18 +7,18 @@ extern void html_raw(const char *txt, size_t size);
 extern void html(const char *txt);
 extern void htmlf(const char *format,...);
 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_path(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);
+extern void html_txt(const char *txt);
+extern void html_ntxt(int len, const char *txt);
+extern void html_attr(const char *txt);
+extern void html_url_path(const char *txt);
+extern void html_url_arg(const char *txt);
+extern void html_hidden(const char *name, const char *value);
+extern void html_option(const char *value, const char *text, const char *selected_value);
+extern void html_link_open(const char *url, const char *title, const char *class);
 extern void html_link_close(void);
 extern void html_fileperm(unsigned short mode);
 extern int html_include(const char *filename);
 
-extern int http_parse_querystring(char *txt, void (*fn)(const char *name, const char *value));
+extern int http_parse_querystring(const char *txt, void (*fn)(const char *name, const char *value));
 
 #endif /* HTML_H */