process: throw TypeError if kill pid not a number

Currently, invalid usage such as:

    process.kill('SIGTERM')
    process.kill(null)
    process.kill(undefined);

all coerce the pid to 0, and signal the current process.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
cluster: enable error/message events using .worker

Between 0.11.1 and 0.11.2, the message and error events stopped
being usable via the cluster.worker object. This commit makes
them usable again. Closes #7998.

Signed-off-by: Fedor Indutny <fedor@indutny.com>