Avoids the case where shell detection would result in
`basename "-bash"` invocation, triggering the error:

    basename: illegal option -- b

fixes #462

`$SHELL` variable is a terrible way of detecting the current shell
because it's not even supposed to reflect the current shell; it's meant
for keeping the value of the default shell for programs to start.

If an explicit `<shell>` argument wasn't passed to `rbenv init`, it
tries to detect the shell by getting the name of its parent process. If
this fails, it falls back on the value of `$SHELL` as before.

Furthermore, `rbenv init` will set the RBENV_SHELL variable in the
current shell to the value of the detected shell so that `sh-shell` and
`sh-rehash` commands don't have to repeat the detection.

If `rbenv init -` outputs `.` in place of `source` and that gets eval'd
by the desktop manager via `~/.profile`, it chokes and prevents the user
from logging in.

Fixes #457

ksh syntax becomes:

    function rbenv {
      typeset command

`typeset` only declares a local variable if there's an explicit
`function` declaration; otherwise the variable leaks.

Other shells use this syntax:

    rbenv() {
      local command

This is for dash compatibility, which supports neither `function` nor
`typeset`.

references #205, fixes #408

Use process subtitution syntax:

    . (rbenv init -|psub)

instead of:

    eval (rbenv init -)

because the latter doesn't work well with newlines.

fixes #444

readlink comes from GNU coreutils.  On systems without it, rbenv used to
spin out of control when it didn't have readlink or greadlink available
because it would re-exec the frontend script over and over instead of the
worker script in libexec.

Fixes #389

This is to insure portability to ksh.

Fixes #205, references #209

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

Fixes #254

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 { ...

Define RBENV_HOME env variable in libexec/rbenv and let all script delegates use this variable to determine rbenv's install location.

Taken from Ryan Tomayko's "GNU is killing Solaris", c.f. http://tomayko.com/writings/gnu-is-killing-solaris

/usr/bin/env seems to have problems with arguments to bash on some
platforms. To bypass this, use set -e instead.