Fix child_process.kill oddities

* When the process is already dead, but the `exit` signal wasn't raised
  yet, the ESRCH error should be ignored.

* When an invalid signal is specified, kill() should throw.

* Like process.kill(), child_process.kill() now preserves a `0` signal
  which can be used to check the liveliness of the child process.

* process.kill() and child_process.kill() will now return true if the
  signal was actually delivered, and false otherwise.

* When an `exec`-ed process is automatically killed because a time or
  buffer limit is exceeded, and the kill() fails, this error should be
  reported through the `exec` callback.

Fixes: #3409