racket/sandbox: use `gui-available?' at sandbox creation

Previously, sandbox creation used `gui?', which is the result of
`gui-available?' at the time that `racket/sandbox' is instanited.
This change makes sandbox behavior less sensitive tothe order in
which modules `require'd into a program are intiantiated.

The change depends on a new `sandbox-make-namespace' default
function for `sandbox-namespace-specs'. The new function uses
either  `make-base-namespace' or `make-gui-namespace', depending
on whether the GUI library is available at that point.

A new `sandbox-gui-enabled' parameter can disable use of the
GUI library even if it is available.

The `gui?' binding is still exported for backward compatibility,
but it shouldn't be used anymore.

original commit: 5630a3a1cab8d7953bfbb290bd6ae63c84ead605

collects/tests/racket/sandbox-gui.rkt

@@ -0,0 +1,48 @@
+#lang racket
+(require racket/sandbox
+         racket/gui/dynamic)
+
+(define-syntax-rule (test expect expr)
+  (do-test 'expr expect expr))
+
+(define (do-test which expect got)
+  (unless (equal? expect got)
+    (error 'test "failed: ~s expect: ~e got: ~e" which expect got)))
+
+;; GUI is initialled allowed by sandbox, but not initially available:
+
+(test #t (sandbox-gui-available))
+(test #f (gui-available?))
+
+;; Create a pre-GUI evaluator:
+
+(define e (call-with-trusted-sandbox-configuration
+           (lambda ()
+             (make-evaluator 'racket))))
+(test (void) (e '(require racket/gui/dynamic)))
+(test #f (e '(gui-available?)))
+
+;; Load GUI library
+
+(test (void) (dynamic-require 'racket/gui #f))
+
+;; Now the GUI is available:
+
+(test #t (gui-available?))
+
+;; Create a post-GUI evaluator:
+
+(define ge (call-with-trusted-sandbox-configuration
+            (lambda ()
+              (make-evaluator 'racket))))
+(test (void) (ge '(require racket/gui/dynamic)))
+(test #t (ge '(gui-available?)))
+
+;; A post-GUI evaluator, but with GUI disabled:
+
+(define pe (parameterize ([sandbox-gui-available #f])
+             (call-with-trusted-sandbox-configuration
+              (lambda ()
+                (make-evaluator 'racket)))))
+(test (void) (pe '(require racket/gui/dynamic)))
+(test #f (pe '(gui-available?)))