process: support symbol events

Event emitters support symbols as event names. The process object
assumes that the event name is a string, and examines the first
three characters to check for signals. This causes an exception
if the event name is a symbol. This commit ensures that the
event name is a string before trying to slice() it.

PR-URL: https://github.com/nodejs/node/pull/4798


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Wyatt Preul <wpreul@gmail.com>