Add a logging subsystem and change all DPRINTF to ISCSI_LOG

Add a mechanism where we can set a logging subsystem that libiscsi can use.
Create an example 'log to stderr' that utilities can use for convenience.

lib/logging.c

@@ -0,0 +1,64 @@
+/*
+   Copyright (C) 2012 by Ronnie Sahlberg <ronniesahlberg@gmail.com>
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as published by
+   the Free Software Foundation; either version 2.1 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program; if not, see <http://www.gnu.org/licenses/>.
+*/
+
+#if defined(WIN32)
+#else
+#include <unistd.h>
+#endif
+
+#include <stdio.h>
+#include <stdarg.h>
+#include <arpa/inet.h>
+#include "iscsi.h"
+#include "iscsi-private.h"
+#include "scsi-lowlevel.h"
+
+void
+iscsi_log_to_stderr(int level, const char *message)
+{
+	fprintf(stderr, "libiscsi:%d %s\n", level, message);
+}
+
+void
+iscsi_set_log_fn(struct iscsi_context *iscsi, iscsi_log_fn fn)
+{
+	iscsi->log_fn = fn;
+}
+
+void
+iscsi_log_message(struct iscsi_context *iscsi, int level, const char *format, ...)
+{
+        va_list ap;
+	static char message[1024];
+	int ret;
+
+	if (iscsi->log_fn == NULL) {
+		return;
+	}
+
+        va_start(ap, format);
+	ret = vsnprintf(message, 1024, format, ap);
+        va_end(ap);
+
+	if (ret < 0) {
+		return;
+	}
+
+	iscsi->log_fn(level, message);
+}
+
+