Validate @recipients before using it for sendmail and Net::SMTP.

Ensure that @recipients is only raw addresses when it is handed to the sendmail
binary OR Net::SMTP, otherwise BCC cases might get an extra <, or wierd stuff
might be passed to the exec.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>