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/func5.tests

@@ -6,8 +6,3 @@ f 2
 
 f() ( echo $1 )
 f 3
-
-f() for i in 1 2 3; do
-	echo $i
-done
-f