Rust's std::env::current_dir and set_current_dir are implemented by calls to getcwd and chdir. The first half in implementing these is to add support to rsix: these should go in rsix::process. There's already a imp/linux_raw implementation of the getcwd and chdir system calls, so this needs a imp/libc implementation which should just call the libc function, and then a public API in src/process. For getcwd, the public API should take care of allocating the buffer and growing it as needed until getcwd successfully writes the whole string, returning an OsString, similar to readlinkat.

The second step is to implement c-scape support in terms of the rsix public API. It may look odd to do it this way, because rsix will be dynamically allocating, and we'll need to copy the (sub)string out of the dynamic allocated buffer into the fixed-size requested buffer. But one of the goals for c-scape is to help test rsix, and to help prepare for rsix itself to be the low-level API instead of c-scape.