Make chmod use std where possible instead of libc; compile for Redox

[ids1024]

No description provided.

[Arcterus]

Does this still work with the setuid, setgid, and sticky bits? I just want to make sure before I merge this because, IIRC, I had to use stuff from libc to get them to work when I first wrote chmod.

[ids1024]

It appears to work.

[Arcterus]

Can you add a note in the help message that umask is not respected on Redox (if it's applicable, not too familiar with Redox atm)?

The build seems to fail on OS X because mode_t is a u16, so casting last_mask to a u32 should fix it.

[ids1024]

I've added a comment to the code about umask on Redox; it isn't currently implemented by Redox.

[Arcterus]

Thanks :)