process: document kill(0), disallow kill(O_RDWR)

The null signal test existed, but only tested the case where the target
process existed, not when it did not exist.

kill(pid, 'O_RDWR'), or any other node constant, "worked". I fixed this
by making kill use the existing isSignal() function to validate its
arguments. Now the signal names supported by process.kill() are the same
as those supported by process.on().