hush: implement negative start in the ${v: -n[:m]} idiom

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-vars/var_bash1a.right b/shell/hush_test/hush-vars/var_bash1a.right
new file mode 100644
index 0000000..1965b5c
--- /dev/null
+++ b/shell/hush_test/hush-vars/var_bash1a.right
@@ -0,0 +1,6 @@
+parameter     'abcdef'
+varoffset2    'cdef'
+varoffset-2   'ef'
+literal '2'   'cdef'
+literal '-2'  'abcdef'
+literal ' -2' 'ef'