http: Insert name into http_post_file

Whereas slcl, the project where slweb started, ignored this field, some
applications might require it.

http.c

@@ -1488,6 +1488,7 @@ static int apply_from_file(struct http_ctx *const h, struct form *const f)
 
     *pf = (const struct http_post_file)
     {
+        .name = f->name,
         .tmpname = f->tmpname,
         .filename = f->filename
     };

include/slweb/http.h

@@ -35,7 +35,7 @@ struct http_payload
 
             const struct http_post_file
             {
-                const char *tmpname, *filename;
+                const char *name, *tmpname, *filename;
             } *files;
         } post;
     } u;