In exceptional cases (custom installation, malfunctions elsewhere), `pyenv-commands --sh` may return nothing.
In non-Fish, this would cause "syntax error near unexpected token `)'" in `pyenv()`.

Bash does not allow to specify a `case` option that would never match.
This works around it by defaulting to `/`. Commands, being filenames, can never match it.
In Fish, nothing needs to be done: it apparently does interpret a `case` without argument as one that never matches.

Fixes breakage in WSL with Pyenv-win installed in the host system;
doesn't affect other supported installation scenarios.

* Support ksh versions

Korn shell had two major versions: ’88 and ’93. Some systems have
ksh installed under the name `ksh93`. A few systems (maybe only
Solaris now) also have a `ksh88`. A few others use the `pdksh` (’88)
or `mksh` (’93) implementations, originated before ksh was open source.

Limit to currently-used versions

---------

Co-authored-by: Kevin Schoedel <kps@datatravelandexperiments.com>

* Remove all use of `greadlink`, fix #2654

Signed-off-by: Harry Chen <i@harrychen.xyz>

* revert greadlink back in tests

---------

Signed-off-by: Harry Chen <i@harrychen.xyz>
Co-authored-by: Anton Petrov <anton.a.petrov@gmail.com>

Co-Authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

Achieves a ~20% speedup.
As it's used in pyenv-latest, this affects all version lookups

* Change other tests to use the newly-created convenience function

* Add test, exclude alphas as well

* Resolve version name that hooks see
* Avoid a 2nd iteration over configured versions

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

In some advanced shell setups, the order of custom-added PATH entries is important.
We disregard it by default, always pushing shims to the front of PATH,
to ensure that Pyenv works even in poorly maintained shell environments
and with minimal hassle for non-export users
(an attempt to do the opposite (#1898) has ended in a disaster).
Some advanced users are however ready and able to carefully maintain their environment
and deal with breakages and inconvenience. This option is for them.