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>

shell/hush_test/hush-arith/arith-postinc.right

@@ -0,0 +1,5 @@
+1 1
+1 1
+1 1
+1 1
+Ok:0

shell/hush_test/hush-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:$?

shell/hush_test/hush-arith/arith.right

@@ -135,6 +135,10 @@ hush: arithmetic syntax error
 hush: arithmetic syntax error
 5 5
 1 1
+6 6
+2 2
+3 3
+1 1
 4 4
 0 0
 hush: arithmetic syntax error

shell/hush_test/hush-arith/arith2.sub

@@ -23,14 +23,14 @@
 echo 5 $(( 4 + ++a ))
 echo 1 $a
 
-# ash doesn't handle it right...
-#ash# echo 6 $(( 4+++a ))
-#ash# echo 2 $a
+# this is treated as 4 + ++a
+echo 6 $(( 4+++a ))
+echo 2 $a
       a=2
 
-# ash doesn't handle it right...
-#ash# echo 3 $(( 4---a ))
-#ash# echo 1 $a
+# this is treated as 4 - --a
+echo 3 $(( 4---a ))
+echo 1 $a
       a=1
 
 echo 4 $(( 4 - -- a ))