src: force line buffering for stderr

SmartOS does not line buffer stderr by default, or at least that is the
behavior on the Node project Jenkins server. Force line buffering. This
resolves the flakiness observed on SmartOS for
test-debug-signal-cluster.

PR-URL: https://github.com/nodejs/node/pull/3701
Fixes: https://github.com/nodejs/node/issues/2476
Refs: https://github.com/nodejs/node/pull/3615


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>