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

... by unsetting `LC_ALL` and passing `LC_CTYPE=C` to `sed`.

This commit fixes issue #1454.

it allows to see functions as well as lines, therefore making it possible to see the control flow

"hyperfine pyenv-versions" before with my bash 4.4:

Time (mean ± σ):    81.7 ms ±   2.2 ms  [User: 67.8 ms, System: 15.7 ms]
Range (min … max):  78.3 ms …  87.2 ms  34 runs

After:

Time (mean ± σ):    70.6 ms ±   2.4 ms  [User: 55.6 ms, System: 16.9 ms]
Range (min … max):  67.9 ms …  78.9 ms  41 runs

"hyperfine pyenv-rehash" before on my bash 4.4:

Time (mean ± σ):   172.8 ms ±   8.2 ms [User: 185.0 ms, System: 24.8 ms]
Range (min … max): 164.2 ms … 198.4 ms 15 runs

After:

Time (mean ± σ):   113.8 ms ±   2.8 ms [User: 127.1 ms, System: 26.1 ms]
Range (min … max): 108.0 ms … 117.6 ms 25 runs

If pyenv doesn't find an executable for the current version/virtualenv,
then search the "system" version as a fallback.

Try for that long no matter what the sleep step available is.

... which is caused by `realpath.dylib` containing illegal UTF-8 byte sequence, and `LC_CTYPE` won't take effect if `LC_ALL` happens to be set to something other than `C`.

This commit fixes issue pyenv/pyenv#1454.

Ref: https://stackoverflow.com/a/23584470

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

* Tweek version(s) help description

Fixes #623

The pyenv help listing offered the "commands" command twice.

## before
```Usage: pyenv <command> [<args>]

Some useful pyenv commands are:
   commands    List all available pyenv commands
   local       Set or show the local application-specific Python version
   global      Set or show the global Python version
   shell       Set or show the shell-specific Python version
   install     Install a Python version using python-build
   uninstall   Uninstall a specific Python version
   rehash      Rehash pyenv shims (run this after installing executables)
   version     Show the current Python version and its origin
   versions    List all Python versions available to pyenv
   which       Display the full path to an executable
   whence      List all Python versions that contain the given executable

See `pyenv help <command>' for information on a specific command.
For full documentation, see: https://github.com/pyenv/pyenv#readme
```

## after
```
Usage: pyenv <command> [<args>]

Some useful pyenv commands are:
   commands    List all available pyenv commands
   local       Set or show the local application-specific Python version
   --version   Display the version of pyenv
   commands    List all available pyenv commands
   exec        Run an executable with the selected Python version
   global      Set or show the global Python version
   help        Display help for a command
   hooks       List hook scripts for a given pyenv command
   init        Configure the shell environment for pyenv
   install     Install a Python version using python-build
   local       Set or show the local application-specific Python version
   prefix      Display prefix for a Python version
   rehash      Rehash pyenv shims (run this after installing executables)
   root        Display the root directory where versions and shims are kept
   shell       Set or show the shell-specific Python version
   shims       List existing pyenv shims
   uninstall   Uninstall a specific Python version
   update      update pyenv and plugins
   version     Show the current Python version and its origin
   version-file   Detect the file that sets the current pyenv version
   version-name   Show the current Python version
   version-origin   Explain how the current Python version is set
   versions    List all Python versions available to pyenv
   whence      List all Python versions that contain the given executable
   which       Display the full path to an executable

See `pyenv help <command>' for information on a specific command.
For full documentation, see: https://github.com/pyenv/pyenv#readme
```

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

This reverts commit d352b4810d.

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

For now the code is using traditional pseudo locking mechanism
based on `noclobber`.