make setenv set -u safe and fix behaviour for no args

dot.mkshrc

@@ -1,9 +1,9 @@
 # $Id$
-# $MirOS: src/bin/mksh/dot.mkshrc,v 1.100 2015/07/10 19:36:33 tg Exp $
+# $MirOS: src/bin/mksh/dot.mkshrc,v 1.101 2015/07/18 23:03:24 tg Exp $
 #-
 # Copyright (c) 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010,
 #		2011, 2012, 2013, 2014, 2015
-#	Thorsten Glaser <tg@mirbsd.org>
+#	mirabilos <tg@mirbsd.org>
 #
 # Provided that these terms and disclaimer and all copyright notices
 # are retained or reproduced in an accompanying document, permission
@@ -384,7 +384,11 @@ function Lstripcom {
 
 # give MidnightBSD's laffer1 a bit of csh feeling
 function setenv {
-	\eval "'export' \"$1\""'="$2"'
+	if (( $# )); then
+		\eval '\export "$1"="${2:-}"'
+	else
+		\typeset -x
+	fi
 }
 
 # toggle built-in aliases and utilities, and aliases and functions from mkshrc