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.

PEP 394 now doesn't require `python` presence or specify what it is.
Tests that invoke it use Py3-specific code.

To facilitate migration

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

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.

The generated script does not take/use any arguments, so passing
/bin/zsh to it serves only to cause confusion.

This reverts commit 438e828eb5.

# Conflicts:
#	test/pyenv_ext.bats

The test "prefix for system in /" is duplicated in test/prefix.bats.
Both tests are completely identical. This commit removes the
duplication.

It appears that the culprit is this merge from rbenv/master, from 2016:

  cf1beda362

With the current development version of bats, this leads to the
following error when running `make test`:

  Error:
  Duplicate test name(s) in file
  "/home/travis/build/pyenv/pyenv/test/prefix.bats":
  test_prefix_for_system_in_-2f

Note that the development version is what pyenv's Makefile uses. With
the latest release of bats, the duplication only leads to a warning:

  bats warning:
  duplicate test name(s) in /src/test/prefix.bats:
  test_prefix_for_system_in_-2f

* Update help docs to clarify multiple version(s) are allowed

* Tweek version(s) help description

Using `exec -a` caused Python to use $PATH to look up the full program name (for
`sys.executable`), which 314937d then tried to fix by changing $PATH
also for the system version.
This is not necessary anymore when not using the short name with `exec`.

This was rejected upstream
(https://github.com/rbenv/rbenv/pull/1089#issuecomment-394531896), since
it is not a problem with Ruby apparently.

Uses $PYENV_ROOT to check if system version is used.

Fixes https://github.com/pyenv/pyenv/issues/98.
Fixes https://github.com/pyenv/pyenv/issues/789.

This was imported semi-automatically in 0965577, but `ruby -S` is
different from `python -S`.

`ruby -S`: look for the script using PATH environment variable
`python -S`: don't imply 'import site' on initialization

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

Ref (abs_dirname in bats): https://github.com/sstephenson/bats/pull/224
Ref: https://github.com/rbenv/rbenv/pull/868

Fixes https://github.com/pyenv/pyenv/issues/580

Command `rbenv version-name > .ruby-version` will create an empty `.ruby-version` file
before running `rbenv-version-file`. This causes `rbenv-version-file` to return empty
string which in turn causes `rbenv-version-name` to return `system`.

Ensure size of `.ruby-version` is non-zero as a workaround.

Otherwise `sed` might not be found later in bats/libexec/bats-exec-test,
in case `/usr/bin` gets removed.

Revert back to original quoting style used before previous commit.
Adjust init.bats to reflect changes for successful tests.

https://travis-ci.org/pyenv/pyenv/jobs/215777826

Keeping rbenv-controlled variables to RBENV_* "namespace" helps with
discoverability (and tools like rbenv-env) but also consistency and a
very minor degree of safety/isolation from env impact.

This ensures that OLD_RBENV_VERSION is never exported. This makes the
implementation a little bit more complex, since more logic needs to be
pushed down into eval'd code.

I need to use release date versioning at least for now until preparing
custom version comparator for brew's formula.