Merge branch 'unistd' into 'master'

include/bits/exec.h

src/platform/Cargo.toml

src/platform/src/lib.rs

src/platform/src/linux/mod.rs

src/platform/src/redox/mod.rs

src/stdlib/src/lib.rs

src/unistd/cbindgen.toml

src/unistd/src/getopt.rs

src/unistd/src/lib.rs

tests/.gitignore

tests/Makefile

tests/exec.c