send-email: quiet some warnings, reject invalid addresses

send-email: quiet some warnings, reject invalid addresses

I'm not sure why we never actually rejected invalid addresses in
the first place.  We just seemed to be using our email validity
checkers to kill duplicates.

Now we just drop invalid email addresses completely and warn
the user about it.

Since we support local sendmail, we'll also accept username-only
addresses.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>