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>

shell/hush_test/hush-vars/param_expand_indicate_error.tests

@@ -1,7 +1,7 @@
 # do all of these in subshells since it's supposed to error out
 
 # first try some invalid patterns
-"$THIS_SH" -c 'echo ${?}'
+#"$THIS_SH" -c 'echo ${?}' -- this is valid as it's the same as $?
 "$THIS_SH" -c 'echo ${:?}'
 
 # then some funky ones