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/prog_log.go

@@ -0,0 +1,33 @@
+package cli
+
+import "github.com/rs/zerolog"
+
+// Debug starts a new message with debug level.
+func (p *prog) Debug() *zerolog.Event {
+	return p.logger.Load().Debug()
+}
+
+// Warn starts a new message with warn level.
+func (p *prog) Warn() *zerolog.Event {
+	return p.logger.Load().Warn()
+}
+
+// Info starts a new message with info level.
+func (p *prog) Info() *zerolog.Event {
+	return p.logger.Load().Info()
+}
+
+// Fatal starts a new message with fatal level.
+func (p *prog) Fatal() *zerolog.Event {
+	return p.logger.Load().Fatal()
+}
+
+// Error starts a new message with error level.
+func (p *prog) Error() *zerolog.Event {
+	return p.logger.Load().Error()
+}
+
+// Notice starts a new message with notice level.
+func (p *prog) Notice() *zerolog.Event {
+	return p.logger.Load().Notice()
+}