bind: refer ubuntu/redhat to add bind user/group

bind: refer ubuntu/redhat to add bind user/group

We refer what ubuntu/redhat did, gave bind a user/group

Here is the example in ubuntu 14.04:

$ ps -eo user,group,cmd | grep "named"
...
bind     bind     /usr/sbin/named -u bind
...

$vim bind9_1%3a9.9.5.dfsg-3_amd64.deb/postinst
...
    # lets give them a bind user/group in all cases.
    getent group bind >/dev/null 2>&1 || addgroup --system bind
    getent passwd bind >/dev/null 2>&1 ||
    adduser --system --home /var/cache/bind --no-create-home \
        --disabled-password --ingroup bind bind
...

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>