Log: add level "DEFAULT"

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

src/Log.hxx

@@ -44,6 +44,11 @@ enum class LogLevel {
 	 */
 	INFO,
 
+	/**
+	 * Interesting informational message.
+	 */
+	DEFAULT,
+
 	/**
 	 * Warning: something may be wrong.
 	 */
@@ -83,6 +88,16 @@ gcc_printf(2,3)
 void
 FormatInfo(const Domain &domain, const char *fmt, ...);
 
+static inline void
+LogDefault(const Domain &domain, const char *msg)
+{
+	Log(domain, LogLevel::DEFAULT, msg);
+}
+
+gcc_printf(2,3)
+void
+FormatDefault(const Domain &domain, const char *fmt, ...);
+
 static inline void
 LogWarning(const Domain &domain, const char *msg)
 {