hush: support "f() (cmd)" functions

Many other shells support this construct

function                                             old     new   delta
parse_stream                                        2950    3018     +68

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

shell/ash_test/ash-misc/func_compound1.tests

@@ -0,0 +1,4 @@
+f() for i in 1 2 3; do
+	echo $i
+done
+f