i#4501: Undo some changes to loader_shared.c made by 9293e7a.

The purpose of 9293e7a was to add TLS support for Windows,
but it broke the use of multiple clients. This patch brings
back the previous code for non-Windows, so on Unix all the
loading happens later, in loader_init_epilogue().

Also add a test of two clients.

Unfortunately this patch undoes the unification of the Unix
and Windows loaders that happened with 9293e7a. It would
be good to find a way of unifying the Unix and Windows loaders
that does not break the use of multiple clients (#6963).

Change-Id: I6bd63de90a1d9eebe50b04ac6117cc3522ae7df7

core/loader_shared.c

@@ -165,6 +165,7 @@ loader_should_call_entry(privmod_t *mod)
 void
 loader_init_prologue(void)
 {
+    /* On Unix this is done (slightly differently) in loader_init_epilogue. */
 #ifdef WINDOWS
     acquire_recursive_lock(&privload_lock);
     VMVECTOR_ALLOC_VECTOR(modlist_areas, GLOBAL_DCONTEXT,