Add user program initproc/user_shell, allow user programs allocate data on heap.

user/src/bin/initproc.rs

@@ -0,0 +1,30 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{
+    fork,
+    wait,
+    exec,
+    yield_,
+};
+
+#[no_mangle]
+fn main() -> i32 {
+    if fork() == 0 {
+        exec("user_shell\0");
+    } else {
+        loop {
+            let mut xstatus: i32 = 0;
+            let pid = wait(&mut xstatus);
+            if pid == -1 {
+                yield_();
+                continue;
+            }
+            println!("[initproc] Release a zombie process!");
+        }
+    }
+    0
+}