Add an unsupervise builtin and function to std::task

Calling task::unsupervise de-parents the current task, with the result
that failures do not propogate up the task tree.

src/lib/task.rs

@@ -2,6 +2,7 @@ native "rust" mod rustrt {
     fn task_sleep(uint time_in_us);
     fn task_yield();
     fn task_join(task t);
+    fn task_unsupervise();
     fn pin_task();
     fn unpin_task();
 }
@@ -23,6 +24,10 @@ fn join(task t) {
     ret rustrt::task_join(t);
 }
 
+fn unsupervise() {
+    ret rustrt::task_unsupervise();
+}
+
 fn pin() {
     rustrt::pin_task();
 }

src/rt/rust_builtin.cpp

@@ -425,6 +425,11 @@ task_join(rust_task *task, rust_task *join_task) {
     }
 }
 
+extern "C" CDECL void
+task_unsupervise(rust_task *task) {
+    task->unsupervise();
+}
+
 /* Debug builtins for std.dbg. */
 
 static void