[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.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances