Implemented the exec builtin

Fixed readline and terminal interactions using stdin instead of /dev/tty

src/procio.rs

@@ -39,6 +39,9 @@ pub enum IoMode {
     buf: String,
     pipe: Arc<OwnedFd>,
   },
+	Close {
+		tgt_fd: RawFd,
+	}
 }
 
 impl IoMode {
@@ -152,6 +155,21 @@ impl<R: Read> IoBuf<R> {
 }
 
 pub struct RedirGuard(IoFrame);
+
+impl RedirGuard {
+	pub fn persist(mut self) {
+		if let Some(saved) = self.0.saved_io.take() {
+			close(saved.0).ok();
+			close(saved.1).ok();
+			close(saved.2).ok();
+		}
+
+		// the guard is dropped here
+		// but since we took the saved fds
+		// the drop does not restore them
+	}
+}
+
 impl Drop for RedirGuard {
   fn drop(&mut self) {
     self.0.restore().ok();