hush: add support for special vars in braces

Some people like to use ${?} rather than $?, so make sure we support all
the special single char vars that use this form.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/shell/hush_test/hush-vars/var1.right b/shell/hush_test/hush-vars/var1.right
index 14b2314..194e7db 100644
--- a/shell/hush_test/hush-vars/var1.right
+++ b/shell/hush_test/hush-vars/var1.right
@@ -1,4 +1,4 @@
 http://busybox.net
 http://busybox.net_abc
-1
-1
+1 1
+1 1