cronwrap: Specify utf8 encoding in mail header

If the output of the external command is not ASCII anyway, the most
likely character set and encoding is unicode/utf8. Say so in the mail
header.

cronwrap/cronwrap

@@ -86,6 +86,8 @@ if ($status || $opt{v})
 			From => "\"cronwrap\@$hostname\" <postmaster\@molgen.mpg.de>",
 			To => \@to,
 			Subject => $subject,
+            'MIME-Version' => '1.0',
+            'Content-Type' => 'text/plain; charset=utf-8',
 		}
 	);