This makes it possible to execute rbenv shims without rbenv's bin
directory in the path.

Before, lines of help that were indented were stripped away.

A command doesn't have to specify Usage docs if it doesn't accept any
arguments. The default usage for a command will be printed as:

    Usage: rbenv ${command}

System ruby is already on PATH (that's the definition of system ruby) and by
duplicating its path by putting it in front, we can break the user's
environment.

Fixes #275

Fixes #119

This is to insure portability to ksh.

Fixes #205, references #209

Docs are comprised from "Usage", "Summary" and "Help" sections, where
"Help" can span multiple commented lines. If it is missing, "Summary" is
shown in its place.

References #204, references #206

Should be `/usr` or `/usr/local`, not `/usr/bin` or `/usr/local/bin`

Fixes #274

Closes #263

Don't have `command -v` abort the whole script prematurely.

Use `rbenv-commands` instead of `rbenv commands`. The latter
unnecessarily goes through `rbenv` executable.

Fixes #254

`rbenv-versions` tries to read the current version to display a marker
next to it, but if that fails the whole script aborts.

This change makes it so that the failures from `rbenv-version-name` are
tolerated. It also makes the `--bare` mode never call it in the first
place, because it doesn't need to display a marker.

This will help with the packaging of rbenv plugin that contain hooks in
Debian.

Added specific message for ksh in identifying the proper shell
initialization file.

Changed rbenv functiond definition to be more portable.
Shell functions should be defined by using the function command or
using the parenthesis grammar, but using both is not portable:

rbenv() {...  -or-
function rbenv { ...

$OLDPWD is a standard shell variable that contains the previous working
directory as set by the "cd" command. No need to save $PWD to some
custom variable.

(We could also have used "cd -" but it prints out $OLDPWD too.)

A trap on the special signal EXIT is executed before the shell
terminates. EXIT actually covers SIGINT and SIGTERM as well, and
we don't need any extra traps for them.

See bash(1) and "help trap" in bash.

Prints $RBENV_ROOT (defaults to ~/.rbenv)

Otherwise, `RBENV_DIR=bin rbenv version-file` loops indefinitely