[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.
Document: cubicweb-docTitle: CubicWeb documentationAuthor: LogilabAbstract: Some base documentation for CubicWeb users and developpersSection: Apps/ProgrammingFormat: HTMLIndex: /usr/share/doc/cubicweb-documentation/index.htmlFiles: /usr/share/doc/cubicweb-documentation/*.html