Introduce __set_errno() throughout libroot.

* add errno_private.h, which defines the __set_errno() macro with
  and without tracing
* instead of setting errno manually, all libroot's code now invokes
  __set_errno(), which makes it much easier to trace changes to errno
* redirect glibc's use of __set_errno() to our own version

headers/private/libroot/errno_private.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2011, Oliver Tappe, [email protected].
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef _ERRNO_PRIVATE_H
+#define _ERRNO_PRIVATE_H
+
+
+#include <errno.h>
+
+//#define TRACE_ERRNO
+
+#if defined(TRACE_ERRNO) && !defined(_KERNEL_MODE)
+#	include <OS.h>
+#	define __set_errno(x) 													  \
+		do { 																  \
+			int error = (x);												  \
+			debug_printf("%s:%d - setting errno to %x\n", __FILE__, __LINE__, \
+				error);														  \
+			errno = error;													  \
+		} while (0)
+#else
+#	define __set_errno(x) \
+		do { errno = (x); } while (0)
+#endif
+
+
+#endif	// _ERRNO_PRIVATE_H