Piping to `head` Causes BrokenPipe

[trytonlux]

toshokan panics running a command like toshokan list | head.

This can be resolved with the #[unix_sigpipe = "sig_dfl"] attribute on fn main().

This is blocked until the unix_sigpipe feature is stabilized.

• Spurious "broken pipe" error messages, when used in typical UNIX shell pipelines rust-lang/rust#46016 (reference)
• Stabilize #[unix_sigpipe = "sig_dfl"] on fn main() rust-lang/rust#120832

[trytonlux]

UI changed for how it'll be enabled (compiler flag)

• Change SIGPIPE ui from #[unix_sigpipe = "..."] to -Zon-broken-pipe=... rust-lang/rust#124480