[net-config] Fixed stopping of mdnsd when dns-sd application terminates abruptly

[net-config] Fixed stopping of mdnsd when dns-sd application terminates abruptly

This patch accepts unique dbus name in handle_launch_mdns() and registers
NameOwnerChanged() signal for all the unique dbus name. Whenever
application terminates abruptly or deinitializes then dbus connection is
closed and net-config receives NameOwnerChanged signal corresponding to
that application and decreases ref count of mdnsd daemon. When ref count
drops to 0 then mdnsd is stopped.

Change-Id: I0d0d5c683ab6ae7b346bec43f70f031a8afee7cf
Signed-off-by: Saurav Babu <saurav.babu@samsung.com>