* 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.

since there proved to be to many cases to fit

Arch Linux has Python is sbin as well as bin

Arch Linux has Python is sbin as well as bin

* 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

The $(type -p | head -1) construct expects to always get a path, so need to ignore such aliases with -P

Closes https://github.com/pyenv/pyenv/issues/1662

* 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

Sometimes it is convenient to be able to temporarily disable something
in a version-file. Because these files often aren't necessarily tracked
in a SCM, especially when working with virtualenvs, the SCM diffs won't
help with showing removed lines which are currently the only way to
disable something.

For example bash-5.1, bash-static.

Added --nosystem argument to skip the system environment when searching for an executable.