Fix recursive dlopen.

The ability to recursively call dlopen is useful for malloc
implementations that wish to load other dynamic modules that
implement reentrant/AS-safe functions to use in their own
implementation.

Given that a user malloc implementation may be called by an
ongoing dlopen to allocate memory the user malloc
implementation interrupts dlopen and if it calls dlopen again
that's a reentrant call.

This patch fixes the issues with the ld.so.cache mapping
and the _r_debug assertion which prevent this from working
as expected.

See:
https://sourceware.org/ml/libc-alpha/2014-12/msg00446.html

dlfcn/moddummy2.c

@@ -0,0 +1,13 @@
+/* Provide a dummy DSO for tst-rec-dlopen to use.  */
+#include <stdio.h>
+#include <stdlib.h>
+
+int
+dummy2 (void)
+{
+  printf ("Called dummy2()\n");
+  /* If the outer dlopen is not dummy1 (becuase of some error)
+     then tst-rec-dlopen will see a value of -1 as the returned
+     result and fail.  */
+  return -1;
+}