Make nanosleep and sleep handle interrupts correctly, plus fix alarm

There are bugs in alarm, nanosleep and sleep that make them behave differently from Linux native. The following fixes are provided:

• alarm was failing to cause SIGALRM because the same pointer was being used for both write and read of the time value
• sleep was not returning the remaining time after EINTR
• nanosleep was not returning remaining time after EINTR (Redox implementation, not tested yet)