unistd: add a preliminary implementation of getopt()

Cargo.lock

src/unistd/Cargo.toml

src/unistd/src/getopt.rs

src/unistd/src/lib.rs

tests/.gitignore

tests/Makefile

tests/expected/unistd/getopt.stderr

tests/expected/unistd/getopt.stdout

tests/unistd/getopt.c