Mask signals during Ergonomic door call

[robertdfrench]

In DOOR_CALL(3C) it says (emphasis mine):

The door_call() function is not a restartable system call. It returns EINTR if a signal was caught and handled by this thread. If the door invocation is not idempotent the caller should mask any signals that may be generated during a door_call() operation.

Or maybe the default door_call should say in the docs that it expects the server procedure to be idempotent, and we could have a separate door_call_nonidempotent call that masks signals to make sure that signals received by the client do not cause the server to be interrupted.