Add mailfrom Defaults option that sets the value of the From:

field in the warning/error mail.  If unset the login name of
the invoking user is used.

logging.c

@@ -510,7 +510,7 @@ send_mail(line)
 
     /* Pipes are all setup, send message via sendmail. */
     (void) fprintf(mail, "To: %s\nFrom: %s\nAuto-Submitted: %s\nSubject: ",
-	def_mailto, user_name, "auto-generated");
+	def_mailto, def_mailfrom ? def_mailfrom : user_name, "auto-generated");
     for (p = def_mailsub; *p; p++) {
 	/* Expand escapes in the subject */
 	if (*p == '%' && *(p+1) != '%') {