glibc.malloc.check: Wean away from malloc hooks

The malloc-check debugging feature is tightly integrated into glibc
malloc, so thanks to an idea from Florian Weimer, much of the malloc
implementation has been moved into libc_malloc_debug.so to support
malloc-check.  Due to this, glibc malloc and malloc-check can no
longer work together; they use altogether different (but identical)
structures for heap management.  This should not make a difference
though since the malloc check hook is not disabled anywhere.
malloc_set_state does, but it does so early enough that it shouldn't
cause any problems.

The malloc check tunable is now in the debug DSO and has no effect
when the DSO is not preloaded.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>