realloc-gnu: Replace module with an alias to realloc-posix.

* tests/test-realloc-posix.c: Renamed from tests/test-realloc-gnu.c. Add
more tests, not yet activated.
* modules/realloc-posix-tests: Renamed from modules/realloc-gnu-tests.
Test test-realloc-posix instead of test-realloc-gnu.
* modules/realloc-gnu: Mark as a deprecated alias of 'realloc-posix'.
* lib/stdlib.in.h (realloc): Ignore GNULIB_REALLOC_GNU.
* modules/stdlib (Makefile.am): Don't substitute GNULIB_REALLOC_GNU,
REPLACE_REALLOC_FOR_REALLOC_GNU.
* m4/stdlib_h.m4 (gl_STDLIB_H_REQUIRE_DEFAULTS): Don't initialize
GNULIB_REALLOC_GNU.
(gl_STDLIB_H_DEFAULTS): Don't initialize
REPLACE_REALLOC_FOR_REALLOC_GNU.
* m4/realloc.m4 (gl_FUNC_REALLOC_GNU): Remove macro.
* m4/malloc.m4 (gl_CHECK_MALLOC_POSIX): Update comment.
* doc/posix-functions/realloc.texi: Remove description of module
'realloc-gnu'.
* MODULES.html.sh (Memory management functions <stdlib.h>): Remove
realloc-gnu. Add realloc-posix.

modules/realloc-posix-tests

@@ -0,0 +1,12 @@
+Files:
+tests/test-realloc-posix.c
+tests/macros.h
+
+Depends-on:
+stdint
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-realloc-posix
+check_PROGRAMS += test-realloc-posix