make libs dir to readable by default for sandboxed code

One example use is the handin server, where the ssl library
is needed to run some programs in my class. (I didn't figure
out exactly why, but it seems reasonable.)

collects/racket/sandbox.rkt

@@ -6,7 +6,8 @@
          racket/string
          syntax/moddep
          racket/gui/dynamic
-         planet/config)
+         planet/config
+         setup/dirs)
 
 (provide gui?
          sandbox-init-hook
@@ -891,6 +892,7 @@
        (read-bytecode ,(PLANET-BASE-DIR))
        (exists ,(find-system-path 'addon-dir))
        (read ,(find-system-path 'links-file))
+       (read ,(find-lib-dir))
        ,@(compute-permissions allow)
        ,@(sandbox-path-permissions))]
     ;; general info