hush: add support for local builtin

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

shell/hush_test/hush-misc/func_local1.right

@@ -0,0 +1,3 @@
+z=a
+z=z
+Done

shell/hush_test/hush-misc/func_local1.tests

@@ -0,0 +1,5 @@
+export z=z
+f() { local z=a; env | grep ^z; }
+f
+env | grep ^z
+echo Done

shell/hush_test/hush-misc/func_local2.right

@@ -0,0 +1,14 @@
+1
+2
+1
+2
+1
+1
+2
+2
+3
+2
+2
+3
+1
+Done

shell/hush_test/hush-misc/func_local2.tests

@@ -0,0 +1,7 @@
+x=1
+f() { echo $x; local x=$((x+1)); echo $x; }
+g() { f; echo $x; f; local x=$((x+1)); f; echo $x; f; }
+f
+g
+echo $x
+echo Done