Auto merge of #715 - goyox86:add-missing-constants-redox, r=alexcrichton

Add some permission and misc (mostly 'mode_t' related) constants to Redox.

Hi!

While porting some software ((exa)[https://github.com/ogham/exa])to Redox I found these missing constants.

Cheers!

//cc @ids1024 @jackpot51