[PATCH] have post_install_network_config use default_name_server and default_name_server_search if non-default are completely empty

[Greg Swift]

the inheritance of the name_servers and name_servers_search do not
behave as I would expect.

Here is an example setup (best viewed in monospace) :

global -> default_name_servers{,_search} have been set

distro -> no settings for these two values
\-> profile -> has settings for name_servers{,_search}
    |   \->   system1  -> inherits from its profile
    |-> sub-profile -> has different settings for name_servers{,_search}
            \->   system2  -> inherits from its profile

Now based on this structure, I would expect system1 to have the
settings from profile and system2 to have the settings from
sub-profile.  However, system2 gets the settings from profile.  If I
remove the settings from profile, system1 no longer has settings, but
system2 gets the settings from sub-profile.

If I remove the settings from both profile and sub-profile, the system
settings are not inherited from the global configuration pool.

I don't know if this is the "best" way to fix this, but I've attached
what, to me, was the most straight forward fix.  I basically patched
the snippet to populate the name_servers and name_servers_search with
values from their respective default value IF they are completely
empty.

If someone would rather iron out the inheritance, I'm up for that.

-greg