hush: fix "export not_yet_defined_var", fix parsing of "cmd | }"

corner case; improve hush_leaktool.sh;
 fix some false positives in testsuite

function                                             old     new   delta
builtin_export                                       191     206     +15
parse_stream                                        2196    2200      +4

shell/hush_test/hush-z_slow/leak_all1.tests

@@ -67,6 +67,7 @@ HERE
     f >/dev/null
     : $((i++))
 done
+unset -f f
 
 memleak
 
@@ -133,6 +134,7 @@ HERE
     f >/dev/null
     : $((i++))
 done
+unset -f f
 
 
 memleak