Revert a revert of a merge that shouldn't have been in master but was intentionally in staging.
Next time I'll do this right after the revert instead of so far down the line...
This reverts commit 9adad8612b.
I can't submit this in smaller units because the various components all
depend on one another during the stdenv bootstrap, so I think this is
the smallest sensible change I can make.
I also removed the symbol-hiding shenanigans in Libsystem. It might mess
up compatibility with 10.9 but I don't really want to support the added
complexity and I see little evidence of anyone else wanting to support
it. If someone cares, we might be able to revive compatibility, but for
now it'll stay like this.
This also changes the versioning scheme to be in more "human-meaningful"
terms, so instead of the internal release numbers we talk about 10.10.5
or 10.9.5.
