(_dl_close): If called recursively, just remember GC needs to be rerun
and decrease l_direct_opencount. Avoid GC if l_direct_opencount
decreased to 1. Rerun GC at the end if any destructor unloaded some
additional libraries.
* elf/Makefile: Add rules to build and run unload6 test.
* elf/unload6.c: New test.
* elf/unload6mod1.c: New file.
* elf/unload6mod2.c: New file.
* elf/unload6mod3.c: New file.
* malloc/hooks.c (mem2chunk_check): Add magic_p argument, set *magic_p
if magic_p is not NULL.
(top_check): Invoke MALLOC_FAILURE_ACTION if MORECORE failed.
(malloc_check): Fail if sz == -1.
(free_check): Adjust mem2chunk_check caller.
(realloc_check): Likewise. Fail if bytes == -1. If bytes == 0 and
oldmem != NULL, call free_check and return NULL. If reallocating
and returning NULL, invert magic byte again to make oldmem valid
region for further checking.
(memalign_check): Fail if bytes == -1.
* malloc/Makefile: Add rules to build and run tst-mcheck.
* malloc/tst-mcheck.c: New test.
