lib/string/sprintf/, lib/, src/, tests/: Move all sprintf(3)-like APIs to a subdirectory

And have a separate file for each pair of APIs.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/prefix_flag.c

@@ -29,7 +29,7 @@
 #endif				/* ENABLE_SUBIDS */
 #include "getdef.h"
 #include "shadowlog.h"
-#include "string/sprintf.h"
+#include "string/sprintf/xasprintf.h"
 
 
 static char *passwd_db_file = NULL;