Clean up internal libc logging.

We only need one logging API, and I prefer the one that does no
allocation and is thus safe to use in any context.

Also use O_CLOEXEC when opening the /dev/log files.

Move everything logging-related into one header file.

Change-Id: Ic1e3ea8e9b910dc29df351bff6c0aa4db26fbb58
diff --git a/linker/linker.h b/linker/linker.h
index d6a4fc5..6196bec 100644
--- a/linker/linker.h
+++ b/linker/linker.h
@@ -36,7 +36,7 @@
 
 #include <link.h>
 
-#include <private/debug_format.h>
+#include "private/libc_logging.h"
 
 #define DL_ERR(fmt, x...) \
     do { \