src: reset signal handler to SIG_DFL on FreeBSD

FreeBSD has a nasty bug with SA_RESETHAND reseting the SA_SIGINFO,
that is in turn set for a libthr wrapper. This leads to a crash.
Work around the issue by manually setting SIG_DFL in the signal
handler.

Fix: https://github.com/joyent/node/issues/9326
PR-URL: https://github.com/iojs/io.js/pull/1218


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>