network/net-lib.sh: delete duplicated DNS items from "/etc/resolv.conf"

Users can pass the DNS information throught "nameserver=" cmdline,
there maybe duplicated inputs.

"/etc/resolv.conf" have some restrictions on the number of DNS items
effective, so make sure that this file contains no duplicated items.

We achieve this by simply making the file have no duplicated lines.

Signed-off-by: Xunlei Pang <xlpang@redhat.com>

modules.d/40network/module-setup.sh

@@ -23,7 +23,7 @@ installkernel() {
 # called by dracut
 install() {
     local _arch _i _dir
-    inst_multiple ip arping dhclient sed
+    inst_multiple ip arping dhclient sed awk
     inst_multiple -o ping ping6
     inst_multiple -o brctl
     inst_multiple -o teamd teamdctl teamnl

modules.d/40network/net-lib.sh

@@ -120,7 +120,7 @@ setup_net() {
     [ -e /tmp/dhclient.$netif.dhcpopts ] && . /tmp/dhclient.$netif.dhcpopts
     # set up resolv.conf
     [ -e /tmp/net.$netif.resolv.conf ] && \
-        cp -f /tmp/net.$netif.resolv.conf /etc/resolv.conf
+        awk '!array[$0]++' /tmp/net.$netif.resolv.conf > /etc/resolv.conf
     [ -e /tmp/net.$netif.gw ]            && . /tmp/net.$netif.gw
 
     # add static route