Kernel: Add (expensive) but valuable userspace symbols to stacks.

This is expensive because we have to page in the entire executable for every
process up front for this to work. This is due to the page fault code not
being strong enough to run while another process is active.

Note that we already had userspace symbols in *crash* stacks. This patch
adds them generally, so they show up in /proc, Process Manager, etc.

There's room for improvement here, but the debugging benefits way overshadow
the performance penalty right now. :^)

Kernel/Process.cpp

@@ -350,6 +350,11 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
         bool success = region->page_in();
         ASSERT(success);
     }
+
+#ifdef EXPENSIVE_USERSPACE_STACKS
+    region->page_in();
+#endif
+
     OwnPtr<ELFLoader> loader;
     {
         // Okay, here comes the sleight of hand, pay close attention..