realpath and relpath implementation

Realpath (#315) provides set of features similar to the plain Debian implementation. GNU options aren't supported yet.
Relpath (#316) options are similar to the old one FreeBSD relpath package, since no coreutils implementation currently providing this command.
I'd prefer if this was spread to multiple lines, like:
```rust
opts.free.iter().map(|x|
    if !resolve_path(x.as_slice(), strip, zero, quiet) {
        retcode = 1;
    }
).last();
```