MFH: r468319

• Pet MDA part, add brackets around Return-Path as per RFC5322 §3.6.6.

Missing brackets lead to a panic when a malformed address line
is fed to dovecot-lda

Submitted by: gahr
Reported by: brnrd via dovecot ML
Obtained from: https://github.com/OpenSMTPD/OpenSMTPD/pull/847/commits/725ba4fa2ddf23bbcd1ff9ec92e86bbfaa6825c8

Approved by: portmgr (adamw)