D031 - The command-line tool does not panic when a pipe closes

println! panics on EPIPE, so selfish imports big.prx | head ended in a backtrace instead of the four lines that were asked for. That is not cosmetic in a binary whose stated purpose is being pointed at real material - paging its output is the normal way to look at it, and it was hit repeatedly while checking this repository's own work.

The usual fix is restoring the default SIGPIPE disposition, which needs unsafe and a libc dependency. This workspace forbids the first and does not want the second for one signal, so output goes through a say! macro that writes to a locked stdout and exits zero when the write fails.

Zero rather than an error: a reader that stopped reading got what it wanted, and a non-zero status would make every | head look like a failed command inside a script.