node: 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
R=@bnoordhuis