Fixed bug in setenv: #f was not handled as deleting the variable.

Also removed superfluous list-copying before call to alist-update as
this has no side effects.

scsh/scsh.scm

@@ -309,15 +309,18 @@
 
 (define (setenv var val)
   (let* ((env (thread-read-env))
-	 (alist (alist-update 
-		 var 
-		 val 
-		 (fold cons '() (env:alist env)))))
+	 (alist (if val
+		    (alist-update 
+		     var 
+		     val 
+		    (env:alist env))
+		    (alist-delete
+		     var
+		     (env:alist env)))))
     (thread-set-env! 
      (make-env 
       #f
-      alist
-      ))))
+      alist))))
 
 (define (alist->env alist)
   (thread-set-env!