fcntl: implement the fcntl function

The fcntl function is currently unimplemented. Both linux and redox support the fcntl syscall.

relevant files:

• src/fcntl/src/lib.rs: Generic platform independent code
• src/platform/<os>/lib.rs: Platform dependent wrapper around the syscall

spec:

• http://pubs.opengroup.org/onlinepubs/7908799/xsh/fcntl.html