Pass a pointer to user_env in to the init_session policy plugin

function so session setup can modify the user environment as needed.
For PAM authentication, merge the PAM environment with the user
environment at init_session time.  We no longer need to swap in the
user_env for environ during session init, nor do we need to disable
the env hooks at init_session time.

src/hooks.c

@@ -278,29 +278,3 @@ deregister_hook(struct sudo_hook *hook)
 
     debug_return_int(rval);
 }
-
-/* Deregister all environment handling hooks. */
-void
-deregister_env_hooks(void)
-{
-    struct sudo_hook_list *tofree;
-    debug_decl(deregister_env_hooks, SUDO_DEBUG_HOOKS)
-
-    while ((tofree = sudo_hook_setenv_list) != NULL) {
-	sudo_hook_setenv_list = sudo_hook_setenv_list->next;
-	efree(tofree);
-    }
-    while ((tofree = sudo_hook_unsetenv_list) != NULL) {
-	sudo_hook_unsetenv_list = sudo_hook_unsetenv_list->next;
-	efree(tofree);
-    }
-    while ((tofree = sudo_hook_getenv_list) != NULL) {
-	sudo_hook_getenv_list = sudo_hook_getenv_list->next;
-	efree(tofree);
-    }
-    while ((tofree = sudo_hook_putenv_list) != NULL) {
-	sudo_hook_putenv_list = sudo_hook_putenv_list->next;
-	efree(tofree);
-    }
-    debug_return;
-}