Remove unnecessary apr_table_do() function casts

Function casts can cause hard-to-debug corruption issues if a
declaration is accidentally changed to be incompatible. Luckily, most of
the function casts for apr_table_do() calls are unnecessary. Remove
them, and adjust the signatures for helpers that weren't taking void* as
the first argument.

The remaining helper that requires a cast is http_filter.c's
form_header_field(), which is probably where many of these casts were
copy-pasted from. I have left it as-is: it has other direct callers
besides apr_table_do(), and it's already documented with warnings not to
change the function signature.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769192 13f79535-47bb-0310-9956-ffa450edef68

modules/http/http_filters.c

@@ -826,8 +826,7 @@ static void fixup_vary(request_rec *r)
      * its comma-separated fieldname values, and then add them to varies
      * if not already present in the array.
      */
-    apr_table_do((int (*)(void *, const char *, const char *))uniq_field_values,
-                 (void *) varies, r->headers_out, "Vary", NULL);
+    apr_table_do(uniq_field_values, varies, r->headers_out, "Vary", NULL);
 
     /* If we found any, replace old Vary fields with unique-ified value */