* 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

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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

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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.

* Document listing available versions in the tutorial (#1677)
* Reflect 3.7.15's extended support
* Document using development head with Homebrew
* Structurize the Upgrading section
* Fix Pyenv-latest documentation

* Fixes #2430 while still preventing CVE-2022-35861
* Adds a skipped version message to stderr

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

* Don't bother reading empty version files
* Implement version file read in pure bash
Is faster with usual sized version files, slower with degenerate cases.

We're going to eliminate the need to mandatorily use `pyenv init --path`.
We can't delete it yet for backward compatibility.
Besides, there's one other use case for it: to enable shims but without
shell integration, e.g. for noninteractive shells.

To be a full-fledged replacement for `pyenv init -` however,
it needs to do rehashing.

Now the setup is to add to both rc and profile:
1) set PYENV_ROOT
   (can do it unconditionally -- since if you change it,
   you need to update all places anyway since any of them can be run first)
2) Add `pyenv` to PATH if not already there
3) eval "$(pyenv init -)"

Not a breaking change, old setup will continue to work.