hush: rework "wait %jobspec" to work in non-interactive shells too

Also add tests. wait5.tests so far fails (but works for ash and dash).

function                                             old     new   delta
builtin_wait                                         305     283     -22

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-misc/wait4.tests b/shell/hush_test/hush-misc/wait4.tests
new file mode 100755
index 0000000..cc34059
--- /dev/null
+++ b/shell/hush_test/hush-misc/wait4.tests
@@ -0,0 +1,2 @@
+sleep 1 | (sleep 1;exit 3) & wait %1
+echo Three:$?