pro: Take the URL to download as a command-line argument

Also, don't print anything other than the download payload to stdout.
This gives us a very simple HTTP download utility :^)

Userland/pro.cpp

@@ -1,21 +1,29 @@
+#include <AK/URL.h>
+#include <LibC/SharedBuffer.h>
 #include <LibCore/CEventLoop.h>
 #include <LibProtocol/Client.h>
-#include <LibC/SharedBuffer.h>
 #include <stdio.h>
 
 int main(int argc, char** argv)
 {
-    (void)argc;
-    (void)argv;
+    if (argc != 2) {
+        printf("usage: %s <url>\n", argv[0]);
+        return 0;
+    }
+
+    String url_string = argv[1];
+    URL url(url_string);
+    if (!url.is_valid()) {
+        fprintf(stderr, "'%s' is not a valid URL\n", url_string.characters());
+        return 1;
+    }
+
     CEventLoop loop;
     auto protocol_client = LibProtocol::Client::construct();
     protocol_client->handshake();
 
-    printf("supports HTTP? %u\n", protocol_client->is_supported_protocol("http"));
-    printf(" supports FTP? %u\n", protocol_client->is_supported_protocol("ftp"));
-
     protocol_client->on_download_finish = [&](i32 download_id, bool success, u32 total_size, i32 shared_buffer_id) {
-        printf("download %d finished, success=%u, shared_buffer_id=%d\n", download_id, success, shared_buffer_id);
+        dbgprintf("download %d finished, success=%u, shared_buffer_id=%d\n", download_id, success, shared_buffer_id);
         if (success) {
             ASSERT(shared_buffer_id != -1);
             auto shared_buffer = SharedBuffer::create_from_shared_buffer_id(shared_buffer_id);
@@ -26,11 +34,11 @@ int main(int argc, char** argv)
     };
 
     protocol_client->on_download_progress = [&](i32 download_id, u32 total_size, u32 downloaded_size) {
-        printf("download %d progress: %u / %u\n", download_id, downloaded_size, total_size);
+        dbgprintf("download %d progress: %u / %u\n", download_id, downloaded_size, total_size);
     };
 
-    i32 download_id = protocol_client->start_download("http://192.168.1.21/");
-    printf("started download with id %d\n", download_id);
+    i32 download_id = protocol_client->start_download(url.to_string());
+    dbgprintf("started download with id %d\n", download_id);
 
     return loop.exec();
 }