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.right b/shell/hush_test/hush-misc/wait4.right
new file mode 100644
index 0000000..f7987db
--- /dev/null
+++ b/shell/hush_test/hush-misc/wait4.right
@@ -0,0 +1 @@
+Three:3