unix: Make sure we signal EOF on the write side when disconnecting

Add a regression test.

Approved by: re (cperciva)
PR: 294014
Reported by: diizzy
Reviewed by: glebius
MFC after: 1 week
Fixes: d15792780760 ("unix: new implementation of unix/stream & unix/seqpacket")
Differential Revision: https://reviews.freebsd.org/D56764

(cherry picked from commit 476805133f5736c2c8638e41d2b5d8dd2c597f3a)
(cherry picked from commit f06697907f360b02682594c6179a7361644d3c87)