Add (partial) support for NetBSD

[iamleot]

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NetBSD does not have any CLOCK_BOOTTIME, treat it like macOS/iOS/tvOS (or probably, more likely, anything that is not Android nor Linux!).

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NOTE: On NetBSD is still not possible to do cargo build --lib --no-default-features --release because there are also several tun bits that are missing. However, as is, this change improve a little the status and is needed for possible boringtun users (that I think do not needs any tun bits) like mitmproxy_rs.

[0-wiz-0]

Can this be merged, please?