Ignore SIGPIPE when serving

[funwithcthulhu]

Summary

• Share the existing SIGPIPE handling between Dream.run and Dream.serve.
• Keep the signal change at the public server entry points rather than changing response writing internals.

Fixes #378.

Validation

• git diff --check
• opam exec -- dune build src/http/http.ml

Blocked locally:

• opam exec -- dune build src/dream.cma src/dream.cmxa needs graphql-lwt, which is not installed in this switch.
• opam exec -- ocamlformat --check src/http/http.ml needs ocamlformat 0.25.1; this switch has 0.29.0.

I did not add a direct regression test here because the reported behavior depends on process-level SIGPIPE handling after a peer closes the socket.