arptables: remove bashism

Bashism:
possible bashism in arptables/etc/init.d/arptables line 14 (should be '.', not 'source'):
source /etc/init.d/functions
possible bashism in arptables/etc/init.d/arptables line 96 ($"foo" should be eval_gettext "foo"):
        echo $"Usage $0 {start|stop|restart|condrestart|save|status}"

Use "." to replace of "source", and change /bin/bash to /bin/sh, the
echo $"foo" works well in our busybox.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>