On all platforms except Linux, calling `withLockIfAvailable()`
recursively just returns `nil`. However, our Linux implementation
chooses to abort the process instead. We don't need this
inconsistent/destructive behaviour and can just return `nil` as we do
elsewhere.
Annotate all of the `Unsafe*` types and `unsafe` functions in the standard
library (including concurrency, synchronization, etc.) as `@unsafe`. Add a
few tests to ensure that we detect uses of these types in clients that
have disabled unsafe code.
