shell: fix arithmentic evaluation of "++7" and such (it is + + 7, i.e. 7)

function                                             old     new   delta
evaluate_string                                      945     988     +43

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

shell/hush_test/hush-arith/arith1.sub

@@ -35,6 +35,6 @@ echo 1 $a
 
 #ash# (( ++ ))
 (  echo $(( +++7 ))  )
-# bash 3.2 apparently thinks that ++ +7 is 7
-#ash# echo $(( ++ + 7 ))
+# ++ is not a inc operator on non-variable, it is the + + sequence
+echo $(( ++ + 7 ))
 #ash# (( -- ))