[Feature request] add a add_header function or an add option

[lublak]

If i look right we can just copy set_header to add_header and ap_table_set to replace ap_table_add
https://github.com/HaxeFoundation/neko/blob/master/libs/mod_neko/cgi.c#L178

Or we can add an option as bool add like in https://github.com/HaxeFoundation/neko/blob/master/libs/mod_tora/mod_tora.c#L132

I think it's better to add a new function so as not to break any code.

If it is added should we also add this function to: https://github.com/HaxeFoundation/haxe/blob/development/std/neko/Web.hx#L108

Also we can add it to php https://github.com/HaxeFoundation/haxe/blob/development/std/php/Web.hx#L129 with header('$h: $v', false);

I am currently trying to find out how Tora is affected.