Allow | head commands from terminal

[KatrionaGoldmann]

Using whatwhat person ... | head returns an error:

Fatal error: exception Sys_error("Broken pipe")

Suspected cause is due to broken pipe signal (see SO question)

We probably want to catch this exception and ignore the error

[yongrenjie]

Digging into this more deeply, you can't even catch the exception and call exit, because exit causes stdout/stderr to be flushed and this flushing triggers the broken pipe error again.

The opam tool has handled this before: ocaml/opam#4216

And the associated PR is probably a good reference of how to fix it: ocaml/opam#4901 The strategy, it seems, is to silently flush stdout and stderr before exiting.