By adding a logger field to "prog" struct, and use this field inside its
method instead of always accessing global mainLog variable. This at
least ensure more consistent usage of the logger during ctrld prog
runtime, and also help refactoring the code more easily in the future
(like replacing the logger library).
On Darwin, modifying /etc/resolv.conf directly does not change interface
network settings. Thus the networksetup command uses to set DNS does not
do anything.
To fix this, after setting DNS using networksetup, re-check the content
of /etc/resolv.conf file to see if the nameservers are what we expected.
Otherwise, re-generate the file with proper nameservers.
