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
