cli: add timeout support for listen command

Currently 'ubus listen' command does not respect the timeout parameter,
i.e., whether timeout parameter is provided, 'ubus listen' will always
run infinitely. This patch let 'ubus listen' command support the timeout
parameter. For example, 'ubus listen -t 60' will cause the command to
exit after 60 seconds.

Signed-off-by: Zhao, Gang <gang.zhao.42@gmail.com>