1. On systems with `readlink -f`, use that to canonicalize the path to libexec directory;
2. Otherwise, resolve symlinks recursively rather than just once.
Considerations:
- `./libexec/rbenv` executable is the entrypoint to the program;
- BASH_SOURCE might be the path to a symlink that has activated `./libexec/rbenv`;
- We must resolve the symlink to learn where rbenv's libexec directory is;
- It's not guaranteed that rbenv commands will always remain directly under their own "libexec" directory, since a package maintainer can change that, e.g. rbenv commands are sometimes placed into `/usr/libexec/rbenv/*`;
- Resolving symlinks might fail and in that case we just assume rbenv project layout.
- Explicitly asking for help with `-h` or `--help` exits with 0 status
and displays help on stdout.
- Not providing any arguments to rbenv results in failure status and
displays version and help on stderr.
