Use #[non_exhaustive] after updating MSRV to 1.40.0

[asomers]

The #[non_exhaustive] attribute, introduced by Rust 1.40.0, should be used for most enums that come from C, since new values could be added by any release. This attribute isn't valuable enough to justify a MSRV bump by itself. But we should still make use of it whenever we update the MSRV for any other reason.

[lucab]

For reference, as of nix 0.8.0 the current MSRV is already at 1.41.0 since #1440.