freebsd-update: restart sshd after upgrade

Sometimes the parent-child sshd protocol changes during an upgrade, and
when this happens sshd will not accept new connections until it is
restarted.

PR: 263489
Reviewed by: kevans, gjb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35109