cmd/cli: avoid accessing mainLog when possible

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).

cmd/cli/resolvconf_windows.go

@@ -6,7 +6,7 @@ import (
 )
 
 // setResolvConf sets the content of resolv.conf file using the given nameservers list.
-func setResolvConf(_ *net.Interface, _ []netip.Addr) error {
+func (p *prog) setResolvConf(_ *net.Interface, _ []netip.Addr) error {
 	return nil
 }