src: add SIGINFO to supported signals

Though not a POSIX signal, SIGINFO is supported by BSD systems
(including Mac OS X) and is amongst the few signals that can be
triggered in a terminal via a simple key combination (CTRL-T).

On Linux, SIGINFO is an alias for SIGPWR; hence the defensive
conditionals in src/node.cc.

PR-URL: https://github.com/nodejs/node/pull/6093


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>