[mail] allow to specify SMTP's MAIL FROM address to config.sendmails(). Closes #3373620
Also modify testlib.Email so we don't loose this information and may test it.
#! /bin/sh -ecase "$1" in remove) if [ -x "/etc/init.d/cubicweb-ctl" ]; then invoke-rc.d cubicweb-ctl stop || true fi ;;esac#DEBHELPER#exit 0