macro: add new helper RET_NERRNO()

This new helper converts libc style syscall return values into
systemd-kernel (actually: kernel style) negative errno values.

It's implemented as macro-like inline function, and propagates return
values >= 0 as themselves and returns -errno for negative error returns.

THis is supposed to be little more than syntactic sugar so that we can
reduce a lot of (short, but still) boilerplate code whever we convert
libc style error handling into our own.

As discussed here:

https://github.com/systemd/systemd/pull/21326#discussion_r748413537

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