hush: add readonly testcase, fix fallout

function                                             old     new   delta
helper_export_local                                  185     214     +29
run_pipe                                            1549    1560     +11
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 40/0)               Total: 40 bytes

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

shell/hush_test/hush-vars/readonly0.right

@@ -0,0 +1,12 @@
+readonly a=A
+readonly b=B
+Ok:0
+hush: a=A: readonly variable
+Fail:1
+hush: a=A: readonly variable
+Fail:1
+hush: a=A: readonly variable
+Fail:1
+Visible:0
+hush: a: readonly variable
+Fail:1

shell/hush_test/hush-vars/readonly0.tests

@@ -0,0 +1,24 @@
+readonly a=A
+b=B
+readonly b
+# readonly on already readonly var is harmless
+readonly b a
+readonly | grep '^readonly [ab]='
+# this should work
+export a b
+export -n a b
+echo Ok:$?
+env | grep -e^a= -e^b=  # shows nothing
+
+# these should all fail (despite the same value being assigned)
+# bash does not abort even in non-interactive more (in script)
+true
+a=A
+echo Fail:$?; true
+readonly a=A
+echo Fail:$?; true
+export a=A
+echo Fail:$?; true
+a=A echo Visible:$? # command still runs
+unset a
+echo Fail:$?; true