Darwin separately stores bits indicating whether a root certificate
should be trusted; this changes Go to read and use those when
initializing SystemCertPool.

Unfortunately, the trust API is very slow. To avoid a delay of up to
0.5s in initializing the system cert pool, we assume that
the trust settings found in kSecTrustSettingsDomainSystem will always
indicate trust. (That is, all root certs Apple distributes are trusted.)
This is not guaranteed by the API but is true in practice.

In the non-cgo codepath, we do not have that benefit, so we must check
the trust status of every certificate. This causes about 0.5s of delay
in initializing the SystemCertPool.

On OS X 10.11 and older, the "security" command requires a certificate
to be provided in a file and not on stdin, so the non-cgo codepath
creates temporary files for each certificate, further slowing initialization.

Updates #18141.

Change-Id: If681c514047afe5e1a68de6c9d40ceabbce54755
Reviewed-on: https://go-review.googlesource.com/33721
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>