bpo-30107: Make SuppressCrashReport quiet on macOS (#1279) (#1335)

bpo-30107: Make SuppressCrashReport quiet on macOS (#1279) (#1335)

On macOS, SuppressCrashReport now redirects /usr/bin/defaults command
stderr into a pipe to not pollute stderr. It fixes a
test_io.test_daemon_threads_shutdown_stderr_deadlock() failure when
the CrashReporter domain doesn't exists. Message logged into stderr:

2017-04-24 16:57:21.432 defaults[41046:2462851]
The domain/default pair of (com.apple.CrashReporter, DialogType) does not exist
(cherry picked from commit d819ad9832292d854e9710493ecdf959b69802e3)