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-postinc.tests b/shell/ash_test/ash-arith/arith-postinc.tests
new file mode 100755
index 0000000..3fd9bfe
--- /dev/null
+++ b/shell/ash_test/ash-arith/arith-postinc.tests
@@ -0,0 +1,5 @@
+echo 1 $((0++1))
+echo 1 $((0--1))
+x=-1; echo 1 $((0-$x))
+x=+1; echo 1 $((0+$x))
+echo Ok:$?