Fix behaviour when stdout gets closed

[AltGr]

Restore the sigpipe handler before exiting, to prevent the finaliser
from crashing on flushes.

Closes #4216: the behaviour on that example is now to exit with code
141, and without an error message.

[kit-ty-kate]

Could you add a one or two line cram test to the testsuite?

[AltGr]

Hmm I can try but we'll have to resort to using shell-pipes and that probably will break on Windows. @dra27 what do you think ?

[kit-ty-kate]

Tests showing the bug added in #5022

[kit-ty-kate]

This needs to be applied for the tests to pass

commit d17de7079cb277081274308fafc4ef9a5afa8164 (HEAD -> broken-pipe)
Author: Kate <kit.ty.kate@disroot.org>
Date:   Wed Jan 26 09:50:03 2022 +0000

    Show that https://github.com/ocaml/opam/issues/4216 is now fixed

diff --git a/tests/cram/unix/list.t b/tests/cram/unix/list.t
index 113085860..99c2fdf0b 100644
--- a/tests/cram/unix/list.t
+++ b/tests/cram/unix/list.t
@@ -1,6 +1,5 @@
 Test https://github.com/ocaml/opam/issues/4216
   $ export OPAMROOT=./opamroot
   $ opam init --bare -n git+https://github.com/ocaml/opam-repository.git#0ec9bbf73a91b8e90b15bb11735859c9552a9888 > /dev/null
-  $ OCAMLRUNPARAM=-b opam list --depends-on dune -s --all-versions | head -n1
+  $ opam list --depends-on dune -s --all-versions | head -n1
   0install.2.14
-  Fatal error: exception Sys_error("Broken pipe")

[AltGr]

Thanks for the rebase/test update @rjbou

[kit-ty-kate]

Thanks!