soccr: Add logging

In order to get messages from libsoccr I propose to
introduce a printing callback that can be set by library
user and that will be called by libsoccr.
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

soccr/soccr.c

 #include "soccr.h"
+
+static void (*log)(unsigned int loglevel, const char *format, ...)
+	__attribute__ ((__format__ (__printf__, 2, 3)));
+static unsigned int log_level = 0;
+
+void libsoccr_set_log(unsigned int level, void (*fn)(unsigned int level, const char *fmt, ...))
+{
+	log_level = level;
+	log = fn;
+}
+
+#define loge(msg, ...) do { if (log && (log_level >= SOCCR_LOG_ERR)) log(SOCCR_LOG_ERR, msg, ##__VA_ARGS__); } while (0)
+#define logd(msg, ...) do { if (log && (log_level >= SOCCR_LOG_DBG)) log(SOCCR_LOG_DBG, msg, ##__VA_ARGS__); } while (0)

soccr/soccr.h

 #ifndef __LIBSOCCR_H__
 #define __LIBSOCCR_H__
+#include <linux/types.h>
+
+struct libsoccr_sk;
+
+void libsoccr_set_log(unsigned int level, void (*fn)(unsigned int level, const char *fmt, ...));
+
+#define SOCCR_LOG_ERR	1
+#define SOCCR_LOG_DBG	2
+
 #endif