crypto/x509: fix and cleanup loadSystemRoots on macOS

crypto/x509: fix and cleanup loadSystemRoots on macOS

Note how untrustedData is never NULL, so loadSystemRoots was checking
the wrong thing.

Also, renamed the C function to CopyPEMRoots to follow the
CoreFoundation naming convention on ownership.

Finally, redirect all debug output to standard error.

Change-Id: Ie80abefadf8974a75c0646aa02fcfcebcbe3bde8
Reviewed-on: https://go-review.googlesource.com/c/go/+/178538
Reviewed-by: Adam Langley <agl@golang.org>