Add subprocesses and convert terminal to an app

src/user/app/echo/echo.c

@@ -0,0 +1,13 @@
+#include "src/lib/string.h"
+#include "src/user/syscall.h"
+
+int main(uint64_t argc, const char **argv) {
+  for (uint64_t i = 1; i < argc; i++) {
+    if (i > 1) {
+      write(1, " ", 1);
+    }
+    write(1, argv[i], string_length(argv[i]));
+  }
+  write(1, "\n", 1);
+  return 0;
+}