Implement allocation buffers for internal use

This commit adds fixed-size allocation buffers.  The primary use
case is in NSS modules, where dynamically sized data is stored
in a fixed-size buffer provided by the caller.

Other uses include a replacement of mempcpy cascades (which is
safer due to the size checking inherent to allocation buffers).

malloc/alloc_buffer_allocate.c

@@ -0,0 +1,36 @@
+/* Allocate a fixed-size allocation buffer using malloc.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library 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.
+
+   The GNU C Library 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 the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <alloc_buffer.h>
+
+#include <stdlib.h>
+
+struct alloc_buffer
+__libc_alloc_buffer_allocate (size_t size, void **pptr)
+{
+  *pptr = malloc (size);
+  if (*pptr == NULL)
+    return (struct alloc_buffer)
+      {
+        .__alloc_buffer_current = __ALLOC_BUFFER_INVALID_POINTER,
+        .__alloc_buffer_end = __ALLOC_BUFFER_INVALID_POINTER
+      };
+  else
+    return alloc_buffer_create (*pptr, size);
+}
+libc_hidden_def (__libc_alloc_buffer_allocate)