shell: handle $((NUM++...) like bash does. Closes 10706

function                                             old     new   delta
evaluate_string                                      680     729     +49

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/ash_test/ash-arith/arith.right b/shell/ash_test/ash-arith/arith.right
index 9b9ca8e..6936f12 100644
--- a/shell/ash_test/ash-arith/arith.right
+++ b/shell/ash_test/ash-arith/arith.right
@@ -126,6 +126,10 @@
 ./arith2.sub: line 5: arithmetic syntax error
 5 5
 1 1
+6 6
+2 2
+3 3
+1 1
 4 4
 0 0
 ./arith2.sub: line 42: arithmetic syntax error