hush: support ${VAR:N:-M}

function                                             old     new   delta
expand_one_var                                      1602    1615     +13
builtin_type                                         114     116      +2

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-vars/var_bash1b.right

@@ -0,0 +1,23 @@
+all    |0123456
+4:     |456
+4:2    |45
+4:-1   |45
+4:-2   |4
+4:-3   |
+-4:    |3456
+-4:2   |34
+-4:-1  |345
+-4:-2  |34
+-4:-3  |3
+-4:-4  |
+-4:i=2 |34
+-4:i=-2|34
+-4:i=-3|3
+-4:i=-4|
+-5:    |23456
+-6:    |123456
+-7:    |0123456
+-8:    |
+-9:    |
+-9:-99 |
+Ok:0