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/log_writer.go

@@ -100,6 +100,7 @@ func (p *prog) initLogging(backup bool) {
 
 	// Initializing internal logging after global logging.
 	p.initInternalLogging(logWriters)
+	p.logger.Store(mainLog.Load())
 }
 
 // initInternalLogging performs internal logging if there's no log enabled.
@@ -108,7 +109,7 @@ func (p *prog) initInternalLogging(writers []io.Writer) {
 		return
 	}
 	p.initInternalLogWriterOnce.Do(func() {
-		mainLog.Load().Notice().Msg("internal logging enabled")
+		p.Notice().Msg("internal logging enabled")
 		p.internalLogWriter = newLogWriter()
 		p.internalLogSent = time.Now().Add(-logWriterSentInterval)
 		p.internalWarnLogWriter = newSmallLogWriter()