Fix import guards in NIOSSHServer and audit sendability

[glbrntt]

The NIOSSHServer example guards all of its code with #if canImport(Foundation.Process) to avoid build failures on platforms without Process.

However, Swift doesn't have submodules like this, Objective-C does. As Foundation transitions to Swift this check now always fails, even if Process is available. The result of this is that example code doesn't compile with newer Swift versions. Removing the can-imports results in a handful of Sendability warnings.