Replace the only `unsafe` usage with appropriate syscall invocation for Config.block_notify_command

[dconnolly]

I am skeptical that using unsafe is the only way to achieve the equivalent operation, if we want to keep it.

https://github.com/ZcashFoundation/zebra/blob/905c225996dbc9435221d5da5ffbf06d5bc00b19/zebra/commands/start.rs#L44

[dconnolly]

Cheers @hdevalence 🙏