Move cwd from kernel to relibc.

src/ld_so/start.rs

src/platform/auxv_defs.rs

src/platform/mod.rs

src/platform/redox/clone.rs

src/platform/redox/exec.rs

src/platform/redox/mod.rs

src/platform/redox/path.rs

src/platform/redox/redox-exec/src/lib.rs

src/start.rs