Convert file descriptors to generic streams

src/user/app/echo/echo.c

@@ -1,7 +1,8 @@
 #include "src/lib/string.h"
+#include "src/lib/util.h"
 #include "src/user/syscall.h"
 
-uint64_t main(uint64_t argc, const char **argv) {
+exit_code_t main(uint64_t argc, const char **argv) {
   for (uint64_t i = 1; i < argc; i++) {
     if (i > 1) {
       write(1, " ", 1);
@@ -9,5 +10,5 @@ uint64_t main(uint64_t argc, const char **argv) {
     write(1, argv[i], string_length(argv[i]));
   }
   write(1, "\n", 1);
-  return 0;
+  return EXIT_CODE_OK;
 }