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.

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.

since there proved to be to many cases to fit

* In some cases (Ubuntu), `pyenv init -` has to be run before `pyenv init --path`.
* The warning has served its purpose by now.

...rather than login mode.

I couldn't get rid of the warning that `pyenv init -` no longer sets path until I did this. It looks like setting only on the login shell wasn't enough to hide the warning in other shells I opened. This fits with [how rbenv does the same thing](https://github.com/rbenv/rbenv/blob/master/libexec/rbenv-init#L74).

I'm way out of my depth here, so someone who knows about shell types should definitely review this.

Closes https://github.com/pyenv/pyenv/issues/1921

* Update install instructions for Bash and Zsh

* Synchronize README.md with `pyenv init`
* Add a ~/.bash_profile note
* Concatenate shims activation into installation for brevity
  (Pyenv can't be used meaningfully without shims anyway)
  Otherwise, we'd need to duplicate all the ~/.profile shenanigans in both sections
* Update based on feedback
* Proofread

Our ~/.rc logic must run after the ~/.profile one

including initial PATH and PYENV_ROOT entries.
The migration hint proved to be unclear to users without them.

E.g. for a GUI session, ~/.profile is executed by the GUI login "shell" at its startup
so one needs to fully log out and log back in.
Before that, the change would only be seen by shells explicitly started as login shells.

To facilitate migration

PATH manipulation should be done in ~/.profile rather than ~/.*rc since .rc can be sourced multiple times

With functions, we have more leeway in what to call

For example bash-5.1, bash-static.

This reverts commit 070e1c859fb2584c3dbe0a6fbe58cbc80938654e, reversing
changes made to 3faeda67bb.

All scripts in libexec/ (excluding pyenv) are called through pyenv,
therefore the shebang lines are not necessary. On some systems this
provides a measurable increase in performance of the shell prompt.

Related to pyenv/pyenv-virtualenv#259

Imported changes from sstephenson/rbenv#453.

modified rbenv source for Python and renamed to pyenv.