fs: make F_OK/R_OK/W_OK/X_OK not writable

Change the fs.F_OK/R_OK/W_OK/X_OK out of fs.js will lead unexpect
exception. Make these properties readonly.

PR-URL: https://github.com/joyent/node/pull/9060


[trev.norris@gmail.com test properties are read only]
Signed-off-by: Trevor Norris <trev.norris@gmail.com>