truly thread-local vms; don't compile popen.scm

* ice-9/Makefile.am: Don't compile popen.scm, its behaviour at runtime
  is not consistent -- seems to miss some GC references? I suspect a bug
  in the compiler. In any case without popen.scm being compiled,
  continuations.test, r4rs.tes, and r5rs_pitfall.test do pass.

* libguile/threads.h (scm_i_thread):
* libguile/threads.c (thread_mark, guilify_self_2): Add a field for the
  thread's vm. Previously I had this as a fluid, but it seems that newly
  created threads share their fluid values from the creator thread; as
  expected, I guess. In any case one VM should not be active in two
  threads.

* libguile/vm.c (scm_the_vm): Change to access the thread-local vm,
  instead of accessing a fluid.
  (scm_the_vm_fluid): Removed.

* module/system/vm/vm.scm: Removed *the-vm*.

libguile/threads.h

@@ -111,6 +111,7 @@ typedef struct scm_i_thread {
   SCM_STACKITEM *continuation_base;
 
   /* For keeping track of the stack and registers. */
+  SCM vm;
   SCM_STACKITEM *base;
   SCM_STACKITEM *top;
   jmp_buf regs;