[zion] Move thread syscalls to the new format.

zion/syscall/syscall.h

@@ -7,3 +7,10 @@ void InitSyscall();
 
 // FIXME: This probably belongs in capability.h
 z_err_t ValidateCap(const RefPtr<Capability>& cap, uint64_t permissions);
+
+#define RET_IF_NULL(expr)    \
+  {                          \
+    if (!expr) {             \
+      return Z_ERR_CAP_TYPE; \
+    }                        \
+  }