Turn DPRINTF into a wrapper to a generic logging function

DPRINTF() made a generic logging function taking a va_list called
vlogger() and reimplemented as a wrapper on top of vlogger().

src/logger.h

@@ -2,6 +2,8 @@
 #ifndef __LOGGER_H__
 #define __LOGGER_H__
 
+#include <stdarg.h>
+
 /* Log domains */
 #define L_CONF    0
 #define L_DAAP    1
@@ -30,6 +32,9 @@
 #define E_SPAM    5
 
 
+void
+vlogger(int severity, int domain, char *fmt, va_list args);
+
 void
 DPRINTF(int severity, int domain, char *fmt, ...);