darwinssl: add support for TLSv1.3

[nickzman]

This is for a feature Apple has publicly announced and documented, but
it's still technically in beta, since it requires Xcode 9.x to build,
and iOS 11 or macOS 10.13 to use the feature. We should probably hold
off on merging this change until the gold masters are released.

[coveralls]

Coverage increased (+0.02%) to 73.168% when pulling 24e76c1 on nickzman/darwinssl-tlsv1.3 into 870d849 on master.

[coveralls]

Coverage increased (+0.02%) to 73.168% when pulling 24e76c1 on nickzman/darwinssl-tlsv1.3 into 870d849 on master.

[bagder]

LGTM!

[jay]

Thanks for your work on this, but it did not land in the appropriate way. It came with two merge commits (which we don't use in master) and the commit message was outdated and lacking a reference. The way to do this in curl project style would be rebase on master and squash the fixup. I force-pushed that just now in 3e492e0, it is identical to your changes but without the other three commits.

[jay]

Thanks for your work on this, but it did not land in the appropriate way. It came with two merge commits (which we don't use in master) and the commit message was outdated and lacking a reference. The way to do this in curl project style would be rebase on master and squash the fixup. I force-pushed that just now in 3e492e0, it is identical to your changes but without the other three commits.