Add a threading syscall API.

zion/capability/capability.cpp

@@ -0,0 +1,17 @@
+#include "capability/capability.h"
+
+template <>
+Process& Capability::obj<Process>() {
+  if (type_ != PROCESS) {
+    panic("Accessing %u cap as object.", type_);
+  }
+  return *static_cast<Process*>(obj_);
+}
+
+template <>
+Thread& Capability::obj<Thread>() {
+  if (type_ != THREAD) {
+    panic("Accessing %u cap as object.", type_);
+  }
+  return *static_cast<Thread*>(obj_);
+}